Type Conversions in JavaScript: Converting Strings to Numbers

There are various ways to convert strings into numbers in JavaScript. We have built-in functions, like parseInt and Number that we can use:

parseInt('42'); // Returns 42
Number('42');   // Returns 42

Both functions return the number equivalent. So the question comes: What is the difference between parseInt and Number, if there's any?

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The Difference Between parseInt and Number

While the Number function simply converts the string into a number, parseInt also uses a second optional argument, a radix. It specifies which numeral system to use. It can be an integer between 2 and 36. By default, we want this to be set to 10 but it is not a default value, so it is recommended to set it explicitly.

parseInt('42', 6);  // Returns 26
parseInt('42', 10); // Returns 42
parseInt('42', 36); // Returns 146

Notice that based on the radix, parseInt will return different values. The Number function on the other hand can work on other values as well, not just on strings:

Number(true);   // Returns 1
Number(false);  // Returns 0
Number(['10']); // Returns 10
Number([1, 2]); // Returns NaN

Note that when using arrays, it will only work with a single element. When Number cannot convert the input value into a number, it will return NaN.

Of course, we can reverse these and convert numbers back to strings, using the String function:

Number('42'); // Returns 42
String(42);   // Returns '42'

But there is an even shorter solution since JavaScript is weakly typed. This means we can convert the two types into one another using a plus sign. It will turn a string into a number:

+'42'   // Results in 42
42 + '' // Results in '42'

As you can see, this works the other way around too. We can convert a number into a string, by simply appending an empty string to the number. Which one to use? It depends on which one you prefer, but make sure you stick with one to stay consistent. For readability, I prefer using the Number object, as it doesn't require a radix. Therefore, it is more reliable even for inexperienced developers.

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Resources:

• JavaScript type conversion
• The Number object
• The parseInt method
• The String object