strict | Bun Docs

Strict mode turns silent mistakes into errors. For example, assigning to a variable that was never declared throws instead of creating a global. ES modules and class bodies are always strict, so a "use strict" directive there does nothing. CommonJS files and plain scripts are only strict if they say so.

In a module, every "use strict" is reported, no matter which option you set. In other files, the option decides where the directive goes. By default a .js file is linted as a module and a .cjs file as CommonJS.

--fix removes directives that do nothing: in a module, in a class, in a function that's already strict, or written twice. It never adds one.

Configure#

Off by default. To turn it on:

bunfig.toml

[lint.rules]
"strict" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"strict" = ["error", "safe"] # "never", "global", "function" or "safe"

Examples#

Reported, because the file is a module:

example.js1 problem

"use strict";

export function run() {
  save();
}

Reported, because a CommonJS file needs the directive at the top:

server.cjs1 problem

const { save } = require("./store.cjs");

save();

example.jsNo problems

export function run() {
  save();
}

In a .cjs file:

server.cjsNo problems

"use strict";

const { save } = require("./store.cjs");

save();

Options#

Option	What it does
`"safe"` (default)	The same as `"global"` in a CommonJS file, and the same as `"function"` in any other script.
`"global"`	Require one directive at the top of the file, and report directives inside functions.
`"function"`	Require a directive in every outermost function, and report one at the top of the file. Use it for scripts that are joined with other scripts, where a directive at the top would apply to all of them.
`"never"`	Report every directive.

With "function" in a .cjs file, functions nested in a strict function don't need their own directive:

widget.cjswith "function"No problems

function setup() {
  "use strict";

  function render() {
    draw();
  }
  render();
}

Here both the directive at the top and setup are reported:

widget.cjswith "function"2 problems

"use strict";

function setup() {
  draw();
}

When not to use it#

If all your files are ES modules, there's nothing to decide. The rule then only catches a leftover "use strict".