Rule: no-promise-as-boolean

Warns for Promises that are used for boolean conditions.

For the most accurate findings, set "strict": true in your tsconfig.json.

It’s recommended to enable the following rules as well:

• strict-boolean-expressions
• strict-type-predicates
• no-floating-promises

Rationale

There are no situations where one would like to check whether a variable’s value is truthy if its type only is Promise. This may only occur when the typings are incorrect or the variable has a union type (like Promise | undefined), of which the latter is allowed.

This rule prevents common bugs from forgetting to ‘await’ a Promise.

Notes:

Config

A list of ‘string’ names of any additional classes that should also be treated as Promises. For example, if you are using a class called ‘Future’ that implements the Thenable interface, you might tell the rule to consider type references with the name ‘Future’ as valid Promise-like types. Note that this rule doesn’t check for type assignability or compatibility; it just checks type reference names.

Config examples

"no-promise-as-boolean": true

"no-promise-as-boolean": [true, {"OPTION_PROMISE_CLASSES": ["Thenable"]}]

Schema

{
  "type": "object",
  "properties": {
    "promise-classes": {
      "type": "array",
      "items": {
        "type": "string"
      }
    }
  }
}

Code examples:

"rules": { "no-promise-as-boolean": true }

async function waiter(custumerDecisionPromise: Promise<any>) {
    if (await custumerDecisionPromise) {
        console.log("Customer ready to take an order.")
    }
}

async function waiter(custumerDecisionPromise: Promise<any>) {
    if (custumerDecisionPromise) {
        console.log("Customer ready to take an order.")
    }
}

"rules": { "no-promise-as-boolean": [true, { "promise-classes": ["CustomPromise"] }] }

function printOrdersPerLine(orderId: number, orderedFoodPromise: CustomPromise<string[]>) {
    orderedFoodPromise.then(orderedFood => {
        console.log(`${orderId} contains the following items:`);

        for (let index = 0; orderedFood; index++) {
            console.log("orderedFood[index]");
        }

        console.log("Done.");
    })
}

function printOrdersPerLine(orderId: number, orderedFoodPromise: CustomPromise<string[]>) {
    console.log(`${orderId} contains the following items:`);

    for (let index = 0; orderedFoodPromise; index++) {
        console.log("orderedFoodPromise[index]");
    }

    console.log("Done.");
}