Why "use strict" improves performance 10x in this example?
Following the question Extending String.prototype performance I am really intrigued, because just adding "use strict"
to a String.prototype
method improved performance 10 times. The explanation by bergi is short and does not explain it to me. Why there is such a dramatic difference between two almost identical methods, that only differ in "use strict"
at the top? Can you explain in more detail and with the theory behind this?
String.prototype.count = function(char) {
var n = 0;
for (var i = 0; i < this.length; i++)
if (this[i] == char) n++;
return n;
};
String.prototype.count_strict = function(char) {
"use strict";
var n = 0;
for (var i = 0; i < this.length; i++)
if (this[i] == char) n++;
return n;
};
// Here is how I measued speed, using Node.js 6.1.0
var STR = '0110101110010110100111010011101010101111110001010110010101011101101010101010111111000';
var REP = 1e4;
console.time('proto');
for (var i = 0; i < REP; i++) STR.count('1');
console.timeEnd('proto');
console.time('proto-strict');
for (var i = 0; i < REP; i++) STR.count_strict('1');
console.timeEnd('proto-strict');
In strict mode, the this
context is not forced to be an object. If you call a function on a non-object, this
will just be that non-object.
In contrast, in non-strict mode, the this
context is always first wrapped in an object if it's not already an object. For example, (42).toString()
first wraps 42
in a Number
object and then calls Number.prototype.toString
with the Number
object as this
context. In strict mode, the this
context is left untouched and just calls Number.prototype.toString
with 42
as this
context.
(function() {
console.log(typeof this);
}).call(42); // 'object'
(function() {
'use strict';
console.log(typeof this);
}).call(42); // 'number'
链接地址: http://www.djcxy.com/p/14996.html
上一篇: Angular捆绑了多少速度?