Array destructuring and spread operator

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It's not 100% clear to me how this piece of code works:

var a = [1, 2, 3];

[x, y, ...a ] = [0, ...a, 4];

// OUTPUT:  [0, 1, 2, 3, 4]

I'm deconstructing the array a using the ... operator.

I am expecting that in second line a bunch of assignments will take place.

The x will be assigned to 0, y will be assigned to ...a (which passes the elements in the array a as individual values).

It's not clear to me, though, how the ...a get assigned to 4. In fact, JS throws an exception when doing:

...a = 4;
// Uncaught SyntaxError: Rest parameter may not have a default initializer

Why does this code output the modified array with the end 4, instead of throwing an exception? How does this work exactly?


It's not clear to me, though, how the ...a get assigned to 4.

It's not.

Lets split things up a little:

On the right hand side of the assignment you are using an array literal with a spread element. The value of a is "flattened" into the new array.

Thus, [0, ...a, 4] is is equivalent to [0].concat(a, [4]). The result is a new array.

var a = [1, 2, 3];

console.log('spread element', [0, ...a, 4]);
console.log('concat', [0].concat(a, [4]));

Spread Operator and Destructuring arrays and objects in JavaScript, Spread syntax allows an iterable such as an array expression or Rest syntax looks exactly like spread syntax, but is used for destructuring  Real world use case for the array spread operator The spread operator can also be used with objects, though it is not a part of the ES6 spec, so you might need to use an extra plugin in your


It is executed like following

var a = [1, 2, 3];
[x, y, ...a ] = [0, ...a, 4];
[x, y, ...a ] = [0, 1, 2, 3, 4];

which means first value in RHS array is assigned to x, second value in RHS array is assigned to y and the remaining values are assigned to a.

Hence, value of x is 0, y is 1 and a is [2, 3, 4]

Destructuring assignment, The Spread syntax allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for  Spread syntax allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected, or an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected.


...a is either equal to .slice(start, end) (left, destructuring) or to .concat(a) (right, spreading):

 [0, ...a, 4]

is equal to:

[0].concat(a).concat([4]) // [0,1,2,3,4]

Whereas:

[x, y, ...a] = array

Is equal to:

x = array[0];
y = array[1];
a = array.slice(2);

Spread syntax, For arrays, you use the spread operator inside the square brackets ( [ ] ). Here, we create a new, shiny array that contains the numbers 1-6. javascript - SPREAD operator in array destructuring - Stack Overflow In the below example, unmapped array items must be mapped to rest parameter in the left hand side, but for some reason the output is different, var planets = ["Mercury", "Earth", "Venus", "Mars",


In the first example, spread ie ... in LHS acts as gatherer whereas on the RHS it acts as spread/rest. IE you are assigning value to variable a when it is on LHS.

var a = [1, 2, 3];

[x, y, ...a ] = [0, ...a, 4];

console.log(a)

Spread and Destructuring: A How-To Guide for JavaScripters, It's not clear to me, though, how the a get assigned to 4 . It's not. Lets split things up a little: On the right hand side of the assignment you are  JavaScript Destructuring Assignment and Spread Operator Jan 15 th , 2016 We will first discussed how destructuring and spread operator can be used in ES6 – in arrays and objects.


Spread Operator or Destructuring, While the spread operator expands an array, object, string or any other and is particularly useful during array and object destructuring. Spread operator in Object Destructuring The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.


Array destructuring and spread operator, operator to copy enumerable properties from one object to another in a more succinct way. The object spread operator is conceptually similar to the ES6 array​  In TypeScript, the spread operator (in form of ellipsis) can be used to initialize arrays and objects from another array or object. You can also use spread operator for object destructuring. Spread Operator vs. apply() Method. The JavaScript’s apply() method calls a function with a given this value, and arguments provided as an array. e.g. In below example, both highlighted function calls are equivalent.


How to use the spread and rest operator, Array destructuring uses an iterator to get to the elements of a source. is used by the spread operator, which constructs and is explained later. I'm new to programming and I've just stumbled upon a problem that I don't fully understand yet. I've looked everywhere for similar problems and their resolution but but I don't see how I can fix