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Converting To Numbers In JavaScript : JavaScript Language Feature - 羡辙杂俎
2013.10.23 15:56:00
Converting To Numbers In JavaScript

Series: JavaScript Language Feature

What you will learn in this post

Number(), parstInt() and parseFloat are three ways to convert to Number type in JavaScript. However, they act differently and may suprise you in some cases. In this post, I would like to talk about what to expect from they and why they act differently.

JavaScript is STRANGE!

Sometimes we think someone or something is strange only because we expect differently.

So, rather than complaining about how strange JavaScript is, why don’t we get to know more about it, and learn what to expect from it? After all, everything and everyone are imperfect in this world, and you can’t just abandon them all and be alone by yourself, not to mention that even yourself may seem strange to others. :cry:

Types In JavaScript

There’re five primitive types in JavaScript, namely Null, Undefined, Boolean, Number and String. Other than that is the Object type, which is called the reference type.

Primitive values are simple pieces of data that are stored on the stack, which is to say that their value is stored directly in the location that the variable accesses. Reference values, on the other hand, are objects that are stored in the heap, meaning that the value stored in the variable location is a pointer to a location in memory where the object is stored.

Professional JavaScript for Web Programmers, Nicholas C. Zakas

When you assign a variable to be primitive type, the value of the variable on the right side is copied. As for reference type, the reference of the variable on the right side is assign to left one.

var a = 32;
var b = a;
b = 16;
console.log(a); // a is still 32


var a = {
    x: 2,
    y: 3
};
var b = a;
b.x = 4;
console.log(a); // a is {x: 4, y: 3}


b = {           // reference b to another object

    u: 5,
    v: 6
}; 
console.log(a); // a is still {x: 4, y: 3}

You may be suprised at String’s being a primitive type if you are from C++ or some other languages. But in JavaScript, once a String is created, it cannot be changed anymore.

var a = "Hello, ";
a += "World!";

In the above code, when executing the second line, the previous value "Hello, " and the new value "World!" are used to create a new String object and make a reference the concatenated result.

Convertion Rules

Number() is the constructor of Number type. With Number("2.3"), we can get a number 2.3. parseInt() converts input to an integer, while parseFloat() to a float number.

+ operator does exactly as Number() when converting.

Professional JavaScript for Web Programmers described the convertion rules in full but you may find it hard to remember all these rules. So, let’s first have a look at them and then I’ll explain about it.

Number()

The Number() function performs conversions based on these rules:

  • When applied to Boolean values, true and false get converted into 1 and 0, respectively.
  • When applied to numbers, the value is simply passed through and returned.
  • When applied to null, Number() returns 0.
  • When applied to undefined, Number() returns NaN.
  • When applied to strings, the following rules are applied:
    • If the string contains only numbers, optionally preceded by a plus or minus sign, it is always converted to a decimal number, so "1" becomes 1, "123" becomes 123, and "011" becomes 11 (note: leading zeros are ignored).
    • If the string contains a valid floating-point format, such as "1.1", it is converted into the appropriate floating-point numeric value (once again, leading zeros are ignored).
    • If the string contains a valid hexadecimal format, such as "0xf", it is converted into an integer that matches the hexadecimal value.
    • If the string is empty (contains no characters), it is converted to 0.
    • If the string contains anything other than these previous formats, it is converted into NaN.
  • When applied to objects, the valueOf() method is called and the returned value is converted based on the previously described rules. If that conversion results in NaN, the toString() method is called and the rules for converting strings are applied.

parseInt()

The parseInt() function examines the string much more closely to see if it matches a number pattern.

  • Leading white space in the string is ignored until the first non–white space character is found.
  • If this first character isn’t a number, the minus sign, or the plus sign, parseInt() always returns NaN, which means the empty string returns NaN (unlike with Number(), which returns 0).
  • If the first character is a number, plus, or minus, then the conversion goes on to the second character and continues on until either the end of the string is reached or a nonnumeric character is found. For instance, "1234blue" is converted to 1234 because "blue" is completely ignored. Similarly, "22.5" will be converted to 22 because the decimal is not a valid integer character.
  • Assuming that the first character in the string is a number, the parseInt() function also recognizes the various integer formats (decimal, octal, and hexadecimal, as discussed previously). This means when the string begins with "0x", it is interpreted as a hexadecimal integer; if it begins with "0" followed by a number, it is interpreted as an octal value.

parseFloat()

The parseFloat() function works in a similar way to parseInt(), looking at each character starting in position 0. It also continues to parse the string until it reaches either the end of the string or a character that is invalid in a floating-point number. This means that a decimal point is valid the first time it appears, but a second decimal point is invalid and the rest of the string is ignored, resulting in "22.34.5" being converted to 22.34.

Another difference in parseFloat() is that initial zeros are always ignored. This function will recognize any of the floating-point formats discussed earlier, as well as the decimal format (leading zeros are always ignored). Hexadecimal numbers always become 0. Because parseFloat() parses only decimal values, there is no radix mode.

A final note: if the string represents a whole number (no decimal point or only a zero after the decimal point), parseFloat() returns an integer.

Professional JavaScript for Web Programmers, Nicholas C. Zakas

OK, I doubt you’ve read that word by word. :stuck_out_tongue:

Well, you don’t have to, because I’m going to explain those confusing rules.

Learn By Examples

Converting string to number is always the most confusing part for all Number(), parseInt() and parseFloat() methods. Although we can infer that the results of passing "2.3" to those functions would be 2.3, 2, and 2.3 respectively, we may probably be wrong when things get complicated.

Float Number

First of all, we want to know when they will return a float number.

In JavaScript, there’s no strict line between an integer and a float number. Numbers are stored as integer if there’s no decimal part. So both console.log(3.0) and console.log(3) output 3.

It’s clear that parseInt() always returns an integer if input is legal while float number for parseFloat(). But what about Number()?

Number("1.2");   // 1.2

Number("2.0");   // 2.0

Number("3");     // 3

Unexpected Characters

There are characters which are not expected in a number (e.g.: "r" and "t" are never expect, while "e" may be part of a scientific notion (but not "ee")). Number(), parseInt() and parseFloat() judge them differently and may result in NaN (stands for Not a Number) or just ignore them. So, let’s try them out one by one.

"e"

Number("9.1e3");        // 9100

parseInt("9.1e3");      // 9

parseFloat("9.1e3");    // 9100

"9.1e3" is a legal form of scientific notion, so we expect JavaScript gives us the correct answer 9100. But unfortunately, parseInt() surprised us with the output of 9. I said that we should learn JavaScript better and know what to expect. So let’s think about why it gives 9.

parseInt() always returns an integer if the input is legal (in the way it thinks). So a simple implementation is to check from the first character whether is a number. If true, loop until the last character which is a number. So parseInt("g34") returns NaN and parseInt("34g") returns 34.

It won’t check if there’s "e" for scientific notion. This is another example of Worse is Better, which argues It is more important for the implementation to be simple than the interface. This makes sense to me. After all, you can still use Number() and parseFloat() in this case.

parseInt() and parseFloat() are similar in that they both ignore from the first illegal character. But "e" is illegal to parseInt() but legal to parseFloat().

This may seems a little confusing, but I believe this is because scientific notion is mostly used as float rather than integer.

Now, I’d like to test the case when "e" is in an illegal position (as defined in scientific notion).

Number("e3");        // NaN

parseInt("e3");      // NaN

parseFloat("e3");    // NaN


Number("3e");        // NaN

parseInt("3e");      // 3

parseFloat("3e");    // 3

When "e" is at the first position, all of them give NaN, which make sense as they’re illegal, although the whole story is more complicated than you might think.

When "e" is at the last position, however, parseInt() and parseFloat() returns 3. It’s easy to understand that of parseInt(), since we’ve discussed above. But it may surprise you a little that parseFloat() also returns 3. In this case, parseFloat() is different from Number() in their judging method. parseFloat() checks until the first illegal (which does not include "e") or last character, while the last "e" falls in Number() is rule of If the string contains anything other than these previous formats, it is converted into NaN, as mentioned in Professional JavaScript for Web Programmers.

In this way, all results of "3e" are the same as that of "3ee3" respectively.

Decimal Point

Number(".3");        // 0.3

parseInt(".3");      // NaN

parseFloat(".3");    // 0.3

Decimal point (.) is legal to Number() and parseFloat(), but illegal to parseInt(). Since there’s no legal character before an illegal one, parseInt() returns NaN.

If you think this is strange, consider the following case:

Number("2.3");       // 2.3

parseInt("2.3");     // 2

parseFloat("2.3");   // 2.3

There’s nothing magic here. But if you think twice, you would probably find parseInt()’s judging algorithm is efficient since it just ignore from the starting illegal character.

Now, what if we abuse decimal point?

Number("2..3");        // NaN

parseInt("2..3");      // 2

parseFloat("2..3");    // 2


Number("2.3.4");       // NaN

parseInt("2.3.4");     // 2

parseFloat("2.3.4");   // 2.3

Just bear it in mind that parseInt() and parseFloat() tends to ignore from the first illegal character while Numbur() checks them completely.

Other Characters

Other illegal characters include "q", "@", "?" and so on.

Number("d");            // NaN

parseInt("d");          // NaN

parseFloat("d");        // NaN


Number("2.3d");         // NaN

parseInt("2.3d");       // 2

parseFloat("2.3d");     // 2.3

I believe you are more confident now.

Hexadecimal and Octal

JavaScript does’t support numeric literals in binary.

// Hexadecimal

Number("0x11");            // 17

parseInt("0x11");          // 17

parseFloat("0x11");        // 0


// Octal

Number("011");             // 11

parseInt("011");           // 11

parseFloat("011");         // 11

"0x11" is recognized as hexadecimal by Number() and parseInt(), but as decimal by parseFloat() (the leading zero is ignored). "011" is not recognized as octal in all cases.

There is a discrepancy between ECMAScript 3 and 5 in regard to using parseInt() with a string that looks like an octal literal. For example:

// 56 (octal) in ECMAScript 3, 0 (decimal) in ECMAScript 5
var num = parseInt("070");

Professional JavaScript for Web Programmers, Nicholas C. Zakas

With parseInt(), you can set the radix explicitly.

parseInt("010", 2);       // 2

parseInt("010", 3);       // 3

parseInt("010", 8);       // 8

parseInt("010", 16);      // 16

In general, Number() supports hexadecimal but not octal; parseInt() recognize in hexadecimal be default but can set the radix explicitly; parseFloat() only support decimal form.

Conclusion

In conclusion, Number() checks the whole string so it’s more strict than the other two methods. parseInt() and parseFloat() stop checking and ignore from the first illegal character, although they may have different judgment on whether a character is legal. Number() and parseFloat() supports scientific notion while parseInt() enables you to set radix explicitly.

There’s no remarkable advantage over the choice of Number(), parseInt() and parseFloat(). You’d better have more information of your input and output form and then, choose a more suitable one for your situation.

I believe the above hasn’t covered all conditions when converting to numbers in JavaScript. But hopefully, it does offer some information on how JavaScript deals with number converting. :yum:

博文对你有帮助吗?如果有的话,要不要送我一本书呢?:heart_eyes:

Series: JavaScript Language Feature


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