没有箭头功能的Typescript / Angular2委托

Question

Why typescript delegates result not equals:

someProperty: any;

someActionsWithItems(item: any) {
   this.someProperty = "test";
}

//if I use this. Its OK.:
this.array.forEach(item => this.someActionsWithItems(item));

// But another. It will be error because context 'this' isn't initialize (Cannot set property 'someProperty' of undefined):
this.array.forEach(this.someActionsWithItems);

Why???

Answer 1

The difference is the arrow function. If you use the arrow function => , it binds this to the function.

In your case

this.array.forEach(item => this.someActionsWithItems(item));

equal to

this.array.forEach(this.someActionsWithItems.bind(this));

You can refer to https://www.w3schools.com/js/js_function_closures.asp

and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions