The usual way to check if the value of a property is the special value undefined, is: if(o.myProperty === undefined) { alert(“myProperty value is the special value `undefined`”); } To check if an object does not actually have such a property, and will therefore return undefined by default when you try to access it: if(!o.hasOwnProperty(‘myProperty’)) … Read more
Well, the problem is that the variable i, within each of your anonymous functions, is bound to the same variable outside of the function. ES6 solution: let ECMAScript 6 (ES6) introduces new let and const keywords that are scoped differently than var-based variables. For example, in a loop with a let-based index, each iteration through … Read more
If you need slightly less control over formatting than the currently accepted answer, Date#toLocaleDateString can be used to create standard locale-specific renderings. The locale and options arguments let applications specify the language whose formatting conventions should be used, and allow some customization of the rendering. Options key examples: day: The representation of the day. Possible … Read more
return false from within a jQuery event handler is effectively the same as calling both e.preventDefault and e.stopPropagation on the passed jQuery.Event object. e.preventDefault() will prevent the default event from occuring, e.stopPropagation() will prevent the event from bubbling up and return false will do both. Note that this behaviour differs from normal (non-jQuery) event handlers, … Read more
Modern HTML5 Techniques for changing classes Modern browsers have added classList which provides methods to make it easier to manipulate classes without needing a library: document.getElementById(“MyElement”).classList.add(‘MyClass’); document.getElementById(“MyElement”).classList.remove(‘MyClass’); if ( document.getElementById(“MyElement”).classList.contains(‘MyClass’) ) document.getElementById(“MyElement”).classList.toggle(‘MyClass’); Unfortunately, these do not work in Internet Explorer prior to v10, though there is a shim to add support for it to IE8 … Read more
Update: ECMAScript 6 (ES6) introduces a new type of literal, namely template literals. They have many features, variable interpolation among others, but most importantly for this question, they can be multiline. A template literal is delimited by backticks: var html = ` <div> <span>Some HTML here</span> </div> `; (Note: I’m not advocating to use HTML … Read more
Do: var isTrueSet = (myValue === ‘true’); using the identity operator (===), which doesn’t make any implicit type conversions when the compared variables have different types. This will set isTrueSet to a boolean true if the string is “true” and boolean false if it is string “false” or not set at all. Don’t: You should … Read more
The method given in the ECMAScript standard to find the class of Object is to use the toString method from Object.prototype. if(Object.prototype.toString.call(someVar) === ‘[object Array]’) { alert(‘Array!’); } Or you could use typeof to test if it is a string: if(typeof someVar === ‘string’) { someVar = [someVar]; } Or if you’re not concerned about … Read more
2017 — 2021 update Since 2009 when this question was asked, JavaScript has evolved significantly. All other answers are now obsolete or overly complicated. Here is the current best practice: function sleep(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } Or as a one-liner: await new Promise(r => setTimeout(r, 2000)); Or const sleep = ms … Read more
The difference is that apply lets you invoke the function with arguments as an array; call requires the parameters be listed explicitly. A useful mnemonic is “A for array and C for comma.” See MDN’s documentation on apply and call. Pseudo syntax: theFunction.apply(valueForThis, arrayOfArgs) theFunction.call(valueForThis, arg1, arg2, …) There is also, as of ES6, the … Read more