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The simple assignment operator (=) assigns a value to a variable, property, or destructuring pattern. It modifies the memory binding of the Left-Hand Side (LHS) expression to point to the evaluated Right-Hand Side (RHS) value. 
LeftHandSideExpression = AssignmentExpression

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Evaluation Mechanics

According to the ECMAScript specification, when the JavaScript engine encounters the = operator, it performs the following sequence of operations:
  1. LHS Resolution: The engine first evaluates the LeftHandSideExpression to resolve its memory reference (an l-value).
  2. RHS Evaluation: The engine then fully evaluates the AssignmentExpression on the right side to produce a value.
  3. Value Binding: The evaluated RHS value is stored in the memory location dictated by the LHS reference.
  4. Return: The operation resolves to the assigned RHS value, making the assignment itself an expression.

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Return Value

Because the assignment operation returns the evaluated RHS value, it can be embedded within larger expressions. 
let y;
// The expression (y = 5) evaluates to 5, which is then added to 10
let x = 10 + (y = 5);

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Associativity

The = operator has right-to-left associativity. When multiple assignment operators appear in the same expression, the JavaScript engine resolves the assignments from right to left. 
let a, b, c;
// Evaluated as: a = (b = (c = 5))
a = b = c = 5;
In this sequence, the reference for a, b, and c are resolved left-to-right. Then, c = 5 is executed, returning 5. That return value is assigned to b, which returns 5, which is finally assigned to a.

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Left-Hand Side Constraints

The LeftHandSideExpression must resolve to a valid reference (a variable, object property, or array element) or a valid destructuring assignment pattern.
  • Invalid Reference: If the LHS is an unresolvable reference or a primitive value, a SyntaxError or ReferenceError is thrown.
// SyntaxError: Invalid left-hand side in assignment
5 = x;
  • Constant Reassignment: If the LHS is a reference bound via the const declaration, the engine throws a TypeError at runtime, as the binding is immutable.
const PI = 3.14;
// TypeError: Assignment to constant variable.
PI = 3.14159;
  • Destructuring: The LHS can be a complex pattern, allowing the = operator to unpack values from arrays or properties from objects into distinct variables.
let a, b, x, y;

[a, b] = [1, 2];
({x, y} = {x: 10, y: 20});
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