Understanding Double Assignment and Operator Precedence in jQuery Code

Hello, I’m ConardLi. While exploring the fact that the world’s largest e‑commerce site Amazon still uses jQuery, I dug into jQuery’s source code and found an intriguing double‑assignment pattern.

Assignment Operations Are Also Expressions

Assigning a value to a variable is an expression whose result is the right‑hand side (RHS) value. For example:

let x
if (x = 1) { // 1 is truthy
  console.log(1) // 1
}

The assignment operator = is right‑associative, so chained assignments work as expected:

let a, b
a = b = 2 // the same as a = (b = 2)
console.log(a) // 2
console.log(b) // 2

Operator Precedence

The jQuery snippet below contains two assignment operators and a property‑access operator: elemData.events = elemData = function(){}; According to the JavaScript operator‑precedence table, property access (precedence 18) outranks assignment (precedence 2). Therefore the expression is evaluated as:

Evaluate elemData.events (property access).

Evaluate the RHS elemData = function(){}, which first assigns the function to elemData and yields that function.

Assign the yielded function to the previously accessed elemData.events.

Code Walk‑through

var elemData = initialValue // 1
// ...
elemData.events = elemData = function(){}; // 2
// ...
elemData.events = {}; // 3

Line 1 simply initializes elemData with initialValue.

Line 2: the old elemData (pointing to initialValue) is overwritten by the function; simultaneously elemData.events receives that function as its value.

Line 3: elemData.events is later replaced with an empty object, discarding the function reference.

This behavior is similar to what happens in a for…in loop when the iterated object is reassigned mid‑loop; the enumeration continues with the original property list until the loop ends.

let obj = {a:1, b:2, c:3}
let obj2 = {d:1, e:2, f:3}
for (const prop in obj) {
  console.log(prop) // a, b, c
  obj = obj2
}
console.log(obj) // {d:1, e:2, f:3}

Practical Application

The same double‑assignment trick can be used to build data structures such as a linked list:

let i = 0, root = {index: i}, node = root
while (i < 10) {
  node.next = node = {} // `node` in `node.next` is the old node
  node.index = ++i   // `node` in `node.index` is the new node
}
node = root
do {
  console.log(node.index) // 0,1,2,...,10
} while ((node = node.next))

The diagram below visualises the same concept.

References

https://www.zhenghao.io/posts/double-assignment

Operator precedence table: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#table