Access to let-bound variable in temporal dead zone¶
ID: js/variable-use-in-temporal-dead-zone
Kind: problem
Security severity:
Severity: error
Precision: very-high
Tags:
- portability
- correctness
Query suites:
- javascript-security-and-quality.qls
Click to see the query in the CodeQL repository
The scope of a variable declared with let is its innermost enclosing block statement, loop or function. Unlike variables declared with var, variables declared with let are not hoisted to the top of their scope, giving rise to a region of code where the variable is in scope, but not declared yet. Accessing a let-bound variable inside this so-called “temporal dead zone” is permitted by some legacy implementations, but is illegal in ECMAScript 2015.
Recommendation¶
Move the let declaration to the beginning of its scope.
Example¶
In the following example, x is initialized before its declaration:
function f() {
x = 23;
let x;
}
The declaration should be moved as follows:
function f() {
let x;
x = 23;
}
References¶
Mozilla Developer Network: Temporal dead zone and errors with let.