+ operator before expression in [removed] what doe

问题:

I was perusing the underscore.js library and I found something I haven't come across before:

if (obj.length === +obj.length) { ... }

What is that + operator doing there? For context, here is a direct link to that part of the file.

回答1:

The unary + operator can be used to convert a value to a number in JavaScript. Underscore appears to be testing that the .length property is a number, otherwise it won't be equal to itself-converted-to-a-number.

回答2:

According to MDN:

The unary plus operator precedes its operand and evaluates to its operand but attempts to converts it into a number, if it isn't already. For example, y = +x takes the value of x and assigns that to y; that is, if x were 3, y would get the value 3 and x would retain the value 3; but if x were the string "3", y would also get the value 3. Although unary negation (-) also can convert non-numbers, unary plus is the fastest and preferred way of converting something into a number, because it does not perform any other operations on the number. It can convert string representations of integers and floats, as well as the non-string values true, false, and null. Integers in both decimal and hexadecimal ("0x"-prefixed) formats are supported. Negative numbers are supported (though not for hex). If it cannot parse a particular value, it will evaluate to NaN.

回答3:

It's a way of ensuring that obj.length is a number rather than a potential string. The reason for this is that the === will fail if the length (for whatever reason) is a string variable, e.g. "3".

回答4:

It's a nice hack to check whether obj.length is of the type number or not. You see, the + operator can be used for string coercion. For example:

alert(+ "3" + 7); // alerts 10

This is possible because the + operator coerces the string "3" to the number 3. Hence the result is 10 and not "37".

In addition, JavaScript has two types of equality and inequality operators:

1. Strict equality and inequality (e.g. 3 === "3" expresses false).
2. Normal equality and inequality (e.g. 3 == "3" expresses true).

Strict equality and inequality doesn't coerce the value. Hence the number 3 is not equal to the string "3". Normal equality and inequality does coerce the value. Hence the number 3 is equal to the string "3".

Now, the above code simply coerces obj.length to a number using the + operator, and strictly checks whether the value before and after the coercion are the same (i.e. obj.length of the type number). It's logically equivalent to the following code (only more succinct):

if (typeof obj.length === "number") {
    // code
}