no-conditional-statement.md

Disallow conditional statements (no-conditional-statement)

This rule disallows conditional statements such as if and switch.

Rule Details

Conditional statements are not a good fit for functional style programming as they are not expressions and do not return a value. Instead consider using the ternary operator which is an expression that returns a value:

For more background see this blog post and discussion in tslint-immutable #54.

Examples of incorrect code for this rule:

/* eslint functional/no-conditional-statement: "error" */

let x;
if (i === 1) {
  x = 2;
} else {
  x = 3;
}

Examples of correct code for this rule:

/* eslint functional/no-conditional-statement: "error" */

const x = i === 1 ? 2 : 3;

/* eslint functional/no-conditional-statement: "error" */

function foo(x, y) {
  return (
    x === y   // if
    ? 0
    : x > y   // else if
    ? 1
    : -1      // else
  );
}

Options

This rule accepts an options object of the following type:

{
  allowReturningBranches: boolean | "ifExhaustive";
}

The default options:

{
  allowReturningBranches: false
}

allowReturningBranches

true

The optional allows conditional statements but only if all defined branches end with a return statement or other terminal. This allows early escapes to be used.

function foo(error, data) {
  if (error) {
    return;
  }

  // ... - Do stuff with data.
}

"ifExhaustive"

This will only allow conditional statements to exists if every case is taken it to account and each has a return statement or other terminal. In other works, every if must have an else and every switch must have a default case. This allows conditional statements to be used like do expressions.

const x = (() => {
  switch(y) {
    case "a":
      return 1;
    case "b":
      return 2;
    default:
      return 0;
})();

Note: Currently this option is not useable with the no-else-return rule; else statements must contain a return statement.