Linting Overview

Flow contains a linting framework that can tell you about more than just type errors. This framework is highly configurable in order to show you the information you want and hide the information you don't.

Configuring Lints in the `.flowconfig` {#toc-configuring-lints-in-the-flowconfig}

Lint settings can be specified in the [lints] section of the .flowconfig as a list of rule=severity pairs. These settings apply globally to the entire project.

[lints]
all=warn
untyped-type-import=error
sketchy-null-bool=off

Configuring Lints from the CLI {#toc-configuring-lints-from-the-cli}

Lint settings can be specified using the --lints flag of a Flow server command as a comma-delimited list of rule=severity pairs. These settings apply globally to the entire project.

flow start --lints "all=warn, untyped-type-import=error, sketchy-null-bool=off"

Configuring Lints with Comments {#toc-configuring-lints-with-comments}

Lint settings can be specified inside a file using flowlint comments. These settings apply to a region of a file, or a single line, or part of a line. For more details see Flowlint Comments.

// flowlint sketchy-null:error
const x: ?number = 0;

if (x) {} // Error

// flowlint-next-line sketchy-null:off
if (x) {} // No Error

if (x) {} /* flowlint-line sketchy-null:off */ // No Error

// flowlint sketchy-null:off
if (x) {} // No Error
if (x) {} // No Error

Lint Settings Precedence {#toc-lint-settings-precedence}

Lint settings in flowlint comments have the highest priority, followed by lint rules in the --lints flag, followed by the .flowconfig. This order allows you to use flowlint comments for fine-grained linting control, the --lints flag for trying out new lint settings, and the .flowconfig for stable project-wide settings.

Within the --lints flag and the .flowconfig, rules lower down override rules higher up, allowing you to write things like

[lints]
# warn on all sketchy-null checks
sketchy-null=warn
# ... except for booleans
sketchy-null-bool=off

The lint settings parser is fairly intelligent and will stop you if you write a redundant rule, a rule that gets completely overwritten, or an unused flowlint suppression. This should prevent most accidental misconfigurations of lint rules.

Severity Levels and Meanings {#toc-severity-levels-and-meanings}

off: The lint is ignored. Setting a lint to off is similar to suppressing a type error with a suppression comment, except with much more granularity.

warn: Warnings are a new severity level introduced by the linting framework. They are treated differently than errors in a couple of ways:

Warnings don't affect the exit code of Flow. If Flow finds warnings but no errors, it still returns 0.
Warnings aren't shown on the CLI by default, to avoid spew. CLI warnings can be enabled by passing the --include-warnings flag to the Flow server or the Flow client, or by setting include_warnings=true in the .flowconfig. This is good for smaller projects that want to see all project warnings at once.

error: Lints with severity error are treated exactly the same as any other Flow error.