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Javascript Articles
Page 522 of 534
What blocks Ruby, Python to get Javascript V8 speed?
Nothing. They can get to V8 speed if proper investments are made in optimizing those language engines as are made for JS by Google in the V8 project.This is all really a matter of how much push is provided to the language by sponsoring organizations to further the development and optimization effort on these languages.
Read MoreJasmine JavaScript Testing - toBe vs toEqual
Arrays can be compared in 2 ways −They refer to the same array object in memory.They may refer to different objects but their contents are all equal.ExampleFor case 1, jasmine provides the toBe method. This checks for reference. For example, describe("Array Equality", () => { it("should check for array reference equility", () => { let arr = [1, 2, 3]; let arr2 = arr // Runs successfully expect(arr).toBe(arr2); // Fails as references are not equal expect(arr).toBe([1, 2, 3]); }); });OutputThis ...
Read MoreIf a DOM Element is removed, are its listeners also removed from memory in javascript?
In modern browsers, if a DOM Element is removed, its listeners are also removed from memory in javascript.Note that this will happen ONLY if the element is reference-free. Or in other words, it doesn't have any reference and can be garbage collected. Only then its event listeners will be removed from memory.
Read MoreWhat is the difference between 'throw new Error' and 'throw someObject' in javascript?
The difference between 'throw new Error' and 'throw someObject' in javascript is that throw new Error wraps the error passed to it in the following format −{ name: 'Error', message: 'Whatever you pass in the constructor' }The throw someObject will throw the object as is and will not allow any further code execution from the try block, ie same as throw new Error.
Read MoreWhat are the differences between Deferreds, Promises and Futures in javascript?
Future is an old term that is same as promise.A promise represents a value that is not yet known. This can better be understood as a proxy for a value not necessarily known when the promise is created.A deferred represents work that is not yet finished. A deferred (which generally extends Promise) can resolve itself, while a promise might not be able to do so. This can also be thought of as a promise that'll always succeed only.A promise is a placeholder for a result which is initially unknown while a deferred represents the computation that results in the value.
Read MoreWhat is the use of sinon.js?
SinonJS provides stand-alone test spies, stubs and mocks. It is a library that we can use to create object mocks for unit testing.Spies − Fake functions that we can use to track executions.Stubs −Functions replacements from which we can return whatever we want or have our functions work in a way that suites us to be able to test multiple scenarios.Mocks −Fake methodsAll these objects help in unit testing our code.
Read MoreHow to create an object property from a variable value in JavaScript?
JS has 2 notations for creating object properties, the dot notation and bracket notation.To create an object property from a variable, you need to use the bracket notation in the following way −Exampleconst obj = {a: 'foo'} const prop = 'bar' // Set the property bar using the variable name prop obj[prop] = 'baz' console.log(obj);OutputThis will give the output −{ a: 'foo', bar: 'baz' }ES6 introduces computed property names, which allow you to do −Exampleconst prop = 'bar' const obj = { // Use a as key a: 'foo', // Use the value of prop ...
Read MoreHow to terminate javascript forEach()?
You can't break from the forEach method and it doesn't provide to escape the loop (other than throwing an exception).You can use other functions like _.find from lodash instead −_.find − it breaks out of the loop when the element is found. For example,Example_.find([1, 2, 3, 4], (element) => { // Check your condition here if (element === 2) { return true; } // Do what you want with the elements here // ... });Throw an exception from forEach. For example,Exampletry { [1, 2, 3, 4].forEach((element) => { // Check your condition here if (element === 2) { throw new Error(); } // Do what you want with the elements here // ... }) } catch (e) { // Do nothing. }
Read MoreHow can I trigger an onchange event manually in javascript?
You can dispatch events on individual elements using the dispatchEvent method. Let's say you have an element test with an onChange event −Event handler −document.querySelector('#test').addEventListener('change', () => console.log("Changed!"))Triggering the event manually −const e = new Event("change"); const element = document.querySelector('#test') element.dispatchEvent(e);This will log the following −Changed!
Read MoreHow to delete a localStorage item when the browser window/tab is closed?
To clear a localStorage data on browser close, you can use the window.onunload event to check for tab close.Let's say you have a local storage object called MyStorage as a global for the sake of this example. Then you can write an event handler −Examplewindow.onunload = () => { // Clear the local storage window.MyStorage.clear() }This will clear the local storage on the tab/window close.
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