Share the strange writing method of JS to determine whether the element is a number_javascript skills

This is what I saw when reading the source code of underscore (1.3.3). Its each method

Copy the code The code is as follows:

var each = _.each = _.forEach = function(obj, iterator, context) {
if (obj == null) return;
if (nativeForEach && obj.forEach === nativeForEach) {
obj.forEach(iterator, context);
} else if (obj.length === obj.length) {
for (var i = 0, l = obj. length; i < l; i ) {
if (iterator.call(context, obj[i], i, obj) === breaker) return;
}
} else {
for (var key in obj) {
if (_.has(obj, key)) {
if (iterator.call(context, obj[key], key, obj) === breaker) return;
}
}
}
};

There is a sentence in this method

Copy code The code is as follows:

if (obj.length === obj.length)

I didn’t understand it after looking at it for a long time. After being pointed out by an expert, this sentence is equivalent to

Copy code The code is as follows:

if (typeof obj.length === 'number')

is used to determine whether the element is of numeric type. typeof and Object.prototype.toString are common ways of writing. The last one is uncommon and difficult for ordinary people to understand.

Some libraries have tool functions for type judgment, such as

Copy code The code is as follows:

function isNumber1(a){
return typeof a === 'number'
}

Or use Object.prototype.toString

Copy code The code is as follows:

function isNumber2(a) {
return Object.prototype.toString.call(a ) === '[object Number]'
}

Changed to this way

Copy code The code is as follows:

function isNumber3(a){
return a === a
}

Test with various types

Copy code The code is as follows:

var arr = ['1', true, false, undefined, null, {}, [], 1]
for (var i=0; iconsole.log(isNumber3(arr[i]))
}

As a result, only the last item in the array is true. That is, only numeric type a === a is true.
Why not use typeof, because string comparison theoretically needs to traverse all characters, and the performance is directly proportional to the length of the string.