Angular what's the meaning of these three dots in @NGRX

What do these three dots mean exactly, and why do I need them?

export function leadReducer(state: Lead[]= [], action: Action {
    switch(action.type){
        case ADD_LEAD:
            return [...state, action.payload];
        case REMOVE_LEAD:
            return state.filter(lead => lead.id !== action.payload.id )
}
}

The three dots are known as the spread operator from Typescript (also from ES7).

The spread operator return all elements of an array. Like you would write each element separately:

let myArr = [1, 2, 3];
return [1, 2, 3];
//is the same as:
return [...myArr];

This is mostly just syntactic sugar as it compiles this:

func(...args);

to this:

func.apply(null, args);

In your case this gets compiled to this:

return [...state, action.payload];
//gets compiled to this:
return state.concat([action.payload]);

It is a spread operator (...) which is used for spreading the element of an array/object or for initializing an array or object from another array or object.

Let's create new array from existing array to understand this.

> let Array1 = [ 1, 2, 3]; //1,2,3 > > let Array2 = [ 4, 5, 6]; //4,5,6 > > //Create new array from existing array > > let copyArray = [...Array1]; > //1,2,3 > > //Create array by merging two arrays > > let mergedArray = [...Array1, ...Array2]; //1,2,3,4,5,6 > > //Create new array from existing array + more elements > > let newArray = [...Array1, 7, 8]; //1,2,3,7,8

The ...(spread operator) works by returning each value from index 0 to index length-1: