Web开发必备：7个实用的JavaScript工具函数

// 返回一个函数，that, as long as it continues to be invoked, will not
// be triggered. The function will be called after it stops being called for
// N milliseconds. If `immediate` is passed, trigger the function on the
// leading edge, instead of the trailing.
function debounce(func, wait, immediate) {
 var timeout;
 return function() {
 var context = this, args = arguments;
 var later = function() {
 timeout = null;
 if (!immediate) func.apply(context, args);
 };
 var callNow = immediate && !timeout;
 clearTimeout(timeout);
 timeout = setTimeout(later, wait);
 if (callNow) func.apply(context, args);
 };
};

// Usage
var myEfficientFn = debounce(function() {
 // All the taxing stuff you do
}, 250);
window.addEventListener('resize', myEfficientFn);

function poll (fn, callback, err, timeout, interval) {
 var startTime = (new Date()).getTime();
 var pi = window.setInterval(function(){
  if (Math.floor(((new Date).getTime() - startTime) / 1000) <= timeout) {
   if (fn()) {
    callback();
   }
  } else {
   window.clearInterval(pi);
   err();
  }
 }, interval)
}

function once(fn, context) { 
 var result;

 return function() { 
 if(fn) {
 result = fn.apply(context || this, arguments);
 fn = null;
 }

 return result;
 };
}

// Usage
var canOnlyFireOnce = once(function() {
 console.log('Fired!');
});

canOnlyFireOnce(); // "Fired!"
canOnlyFireOnce(); // nada

var getAbsoluteUrl = (function() {
 var a;

 return function(url) {
 if(!a) a = document.createElement('a');
 a.href = url;

 return a.href;
 };
})();

// Usage
getAbsoluteUrl('/something'); 

;(function() {

 // Used to resolve the internal `[[Class]]` of values
 var toString = Object.prototype.toString;
 
 // Used to resolve the decompiled source of functions
 var fnToString = Function.prototype.toString;
 
 // Used to detect host constructors (Safari > 4; really typed array specific)
 var reHostCtor = /^\[object .+?Constructor\]$/;

 // Compile a regexp using a common native method as a template.
 // We chose `Object#toString` because there's a good chance it is not being mucked with.
 var reNative = RegExp('^' +
 // Coerce `Object#toString` to a string
 String(toString)
 // Escape any special regexp characters
 .replace(/[.*+?^${}()|[\]\/\\]/g, '\\$&')
 // Replace mentions of `toString` with `.*?` to keep the template generic.
 // Replace thing like `for ...` to support environments like Rhino which add extra info
 // such as method arity.
 .replace(/toString|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?') + '$'
 );
 
 function isNative(value) {
 var type = typeof value;
 return type == 'function'
  // Use `Function#toString` to bypass the value's own `toString` method
  // and avoid being faked out.
  ? reNative.test(fnToString.call(value))
  // Fallback to a host object check because some environments will represent
  // things like typed arrays as DOM methods which may not conform to the
  // normal native pattern.
  : (value && type == 'object' && reHostCtor.test(toString.call(value))) || false;
 }
 
 // export however you want
 module.exports = isNative;
}());

// Usage
isNative(alert); // true
isNative(myCustomFunction); // false

// Build a better Sheet object 
Sheet = (function() {
 // Build style
 var style = document.createElement('style');
 style.setAttribute('media', 'screen');
 style.appendChild(document.createTextNode(''));
 document.head.appendChild(style);

 // Build and return a single function
 return function(rule){ style.sheet.insertRule( rule, style.sheet.cssRules.length ); } ;
})();

// Then call as a function
Sheet(".stats { position: relative ; top: 0px }") ;

function matchesSelector(el, selector) {
 var p = Element.prototype;
 var f = p.matches || p.webkitMatchesSelector || p.mozMatchesSelector || p.msMatchesSelector || function(s) {
 return [].indexOf.call(document.querySelectorAll(s), this) !== -1;
 };
 return f.call(el, selector);
}

// Usage
matchesSelector(document.getElementById('myDiv'), 'div.someSelector[some-attribute=true]')