eslint/block-scoped-var Suspicious

What it does

Enforces that variables are both declared and used within the same block scope. This rule prevents accidental use of variables outside their intended block, mimicking C-style block scoping in JavaScript.

Why is this bad?

JavaScript’s var declarations are hoisted to the top of their enclosing function, which can cause variables declared in a block (e.g., inside an if or for) to be accessible outside of it. This can lead to hard-to-find bugs. By enforcing block scoping, this rule helps avoid hoisting issues and aligns more closely with how other languages treat block variables.

Options

No options available for this rule.

Examples

Examples of incorrect code for this rule:

/* block-scoped-var: "error" */

function doIf() {
  if (true) {
    var build = true;
  }
  console.log(build);
}

function doLoop() {
  for (var i = 0; i < 10; i++) {
    // do something
  }
  console.log(i); // i is accessible here
}

function doSomething() {
  if (true) {
    var foo = 1;
  }
  if (false) {
    foo = 2;
  }
}

function doTry() {
  try {
    var foo = 1;
  } catch (e) {
    console.log(foo);
  }
}

Examples of correct code for this rule:

/* block-scoped-var: "error" */

function doIf() {
  var build;
  if (true) {
    build = true;
  }
  console.log(build);
}

function doLoop() {
  var i;
  for (i = 0; i < 10; i++) {
    // do something
  }
  console.log(i);
}

function doSomething() {
  var foo;
  if (true) {
    foo = 1;
  }
  if (false) {
    foo = 2;
  }
}

function doTry() {
  var foo;
  try {
    foo = 1;
  } catch (e) {
    console.log(foo);
  }
}

How to use

To enable this rule in the CLI or using the config file, you can use:

CLI

oxlint --deny block-scoped-var

Config (.oxlintrc.json)

{
  "rules": {
    "block-scoped-var": "error"
  }
}

References

• Rule Source