init-declarations

A variable declared without a value is undefined until something assigns to it. If one path through the code skips the assignment, later code reads undefined without any error. Giving every variable a value where it's declared closes that gap.

Configure

Off by default. To turn it on:

[lint.rules]
"init-declarations" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"init-declarations" = ["error", "always"] # "always" or "never"

Examples

Reported:

let total;
if (cart.items.length > 0) {
  total = sum(cart.items);
}

Not reported:

let total = 0;
if (cart.items.length > 0) {
  total = sum(cart.items);
}

The variable in a for...in or for...of loop gets its value from the loop, so it counts as initialized:

for (let item of cart.items) {
  render(item);
}

In TypeScript, a declare statement can't have a value, so it isn't reported:

declare let version: string;

Options

[lint.rules]
init-declarations = ["error", "never", { ignoreForLoopInit = true }]

let count = 0;

let count;
count = 0;

const limit = 10;

ignoreForLoopInit

for (let i = 0; i < 3; i++) {
  retry();
}

for (let item of cart.items) {
  render(item);
}

for (let i = 0; i < 3; i++) {
  retry();
}

for (let item of cart.items) {
  render(item);
}