Javascript – What Typescript type is Angular2 event

angularjavascripttypescript

If I have the following button in an html file

<button (click)="doSomething('testing', $event)">Do something</button>

Also, in the corresponding component, I have this function

doSomething(testString: string, event){
    event.stopPropagation();
    console.log(testString + ': I am doing something');
}

Is there a proper type that should be assigned to the $event input?
The event parameter itself is an object, BUT if I assign it to a type object, I get an error

Property 'stopPropogation' does not exist on type object

So, what does Typescript consider the $event input?

Best Answer

As suggested by @AlexJ

The event you passed through $event is a DOM event, therefore you can use the EventName as the type.

In your case this event is a MouseEvent, and the docs says, quoting

The MouseEvent interface represents events that occur due to the user interacting with a pointing device (such as a mouse). Common events using this interface include click, dblclick, mouseup, mousedown.

In all those cases you'll get a MouseEvent.

Another example : if you have this code

<input type="text" (blur)="event($event)"

When the event triggers you'll get a FocusEvent.

So you can do it really simple, console log the event and you'll see a message similar to this one that'll we have the event name

FocusEvent {isTrusted: true, relatedTarget: null, view: Window, detail: 0, which: 0…}

You can always visit the docs for a list of existing Events.

Edit

You can also check for TypeScript dom.generated.d.ts with all the typings ported. In your case stopPropagation() is part of Event, extended by MouseEvent.

Native deep cloning

It's called "structured cloning", works experimentally in Node 11 and later, and hopefully will land in browsers. See this answer for more details.

Fast cloning with data loss - JSON.parse/stringify

If you do not use Dates, functions, undefined, Infinity, RegExps, Maps, Sets, Blobs, FileLists, ImageDatas, sparse Arrays, Typed Arrays or other complex types within your object, a very simple one liner to deep clone an object is:

JSON.parse(JSON.stringify(object))

const a = {
  string: 'string',
  number: 123,
  bool: false,
  nul: null,
  date: new Date(),  // stringified
  undef: undefined,  // lost
  inf: Infinity,  // forced to 'null'
  re: /.*/,  // lost
}
console.log(a);
console.log(typeof a.date);  // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date);  // result of .toISOString()

See Corban's answer for benchmarks.

Reliable cloning using a library

Since cloning objects is not trivial (complex types, circular references, function etc.), most major libraries provide function to clone objects. Don't reinvent the wheel - if you're already using a library, check if it has an object cloning function. For example,

lodash - cloneDeep; can be imported separately via the lodash.clonedeep module and is probably your best choice if you're not already using a library that provides a deep cloning function
AngularJS - angular.copy
jQuery - jQuery.extend(true, { }, oldObject); .clone() only clones DOM elements
just library - just-clone; Part of a library of zero-dependency npm modules that do just do one thing. Guilt-free utilities for every occasion.

ES6 (shallow copy)

For completeness, note that ES6 offers two shallow copy mechanisms: Object.assign() and the spread syntax. which copies values of all enumerable own properties from one object to another. For example:

var A1 = {a: "2"};
var A2 = Object.assign({}, A1);
var A3 = {...A1};  // Spread Syntax

Javascript – the !! (not not) operator in JavaScript

Converts Object to boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true.

!oObject  // inverted boolean
!!oObject // non inverted boolean so true boolean representation

So !! is not an operator, it's just the ! operator twice.

Real World Example "Test IE version":

const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);  
console.log(isIE8); // returns true or false

If you ⇒

console.log(navigator.userAgent.match(/MSIE 8.0/));  
// returns either an Array or null

But if you ⇒

console.log(!!navigator.userAgent.match(/MSIE 8.0/));  
// returns either true or false

Best Answer

Related Solutions

Javascript – the most efficient way to deep clone an object in JavaScript

Native deep cloning

Fast cloning with data loss - JSON.parse/stringify

Reliable cloning using a library

ES6 (shallow copy)

Javascript – the !! (not not) operator in JavaScript

Related Topic