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--customConditions takes a list of additional conditions that should succeed when TypeScript resolves from an [exports] or (https://nodejs.org/api/packages.html#exports) or imports field of a package.json. These conditions are added to whatever existing conditions a resolver will use by default.

For example, when this field is set in a tsconfig.json as so:

jsonc

{
    "compilerOptions": {
        "target": "es2022",
        "moduleResolution": "bundler",
        "customConditions": ["my-condition"]
    }
}

{
    "compilerOptions": {
        "target": "es2022",
        "moduleResolution": "bundler",
        "customConditions": ["my-condition"]
    }
}

Any time an exports or imports field is referenced in package.json, TypeScript will consider conditions called my-condition.

So when importing from a package with the following package.json

jsonc

{
    // ...
    "exports": {
        ".": {
            "my-condition": "./foo.mjs",
            "node": "./bar.mjs",
            "import": "./baz.mjs",
            "require": "./biz.mjs"
        }
    }
}

{
    // ...
    "exports": {
        ".": {
            "my-condition": "./foo.mjs",
            "node": "./bar.mjs",
            "import": "./baz.mjs",
            "require": "./biz.mjs"
        }
    }
}

TypeScript will try to look for files corresponding to foo.mjs.

This field is only valid under the node16, nodenext, and bundler options for --moduleResolution.