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. See ES modules and CommonJS for how bun lint tells them apart.

--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".