javascript - What is happening when using the keyword this in lambdas vs functions with Typescript -

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• how access correct `this` inside callback? 6 answers

i using typescript angular 2 project. notice when use keyword this inside labmda expression vs function, this refers different things.

for example, let's have angular component following.

export class mycomponet {  isactive = true;  names = [ "john", "jeff", "jared" ];   dosomethingwithlambda() {   names.foreach( (value, index) => {    if(this.isactive) { //this.isactive refers mycomponent.isactive     //do something...    }   });  }   dosomethingwithfunction() {   names.foreach( function(value, index) {    if(this.isactive) { //this.isactive undefined, since refers function     //do    }   });  }   dosomethingwithfunction2() {   let isactive = this.isactive;   names.foreach( function(value, index) {    if(isactive) { //if change isactive change mycomponent.isactive?     //do    }   });  } } 

what happening here (behind scene, speak)? what's magic behind this inside lambda makes able "correctly" refer outer class' fields? understand this inside function refer function itself.

also, have dosomethingwithfunction2 method reference mycomponent.isactive local variable. if change local variable, should changing 1 references, right? (regardless of being "primitive" integer/number or "object" json { })

the fat-arrow function syntax shorthand for:

function () { }.bind(this); 

bind javascript es5 method equivalent this:

function.prototype.bind = function bind(context) {   var func = this;   return function () {     return func.apply(context, arguments);   }; } 

in regards

also, have dosomethingwithfunction2 method reference mycomponent.isactive local variable. if change local variable, should changing 1 references, right? (regardless of being "primitive" integer/number or "object" json { })

in javascript, variables pointers , except limited cases (primitives , copy-on-write objects) change referenced value when mutated. reassigning not change original value, e.g. isactive = false; this.isactive = false in fact re-assign variable isactive on this correctly assigned.