# E0057: use of undeclared variable

It is an error to use a function or variable without declaring it:

    consol.write("Hello, world!");
    const one = Math.sin(pi/2);

    function encrypt() {
      return data ^ 0x13371337;
    }

    if (isHappy) {
      let emotion = "happy";
    } else {
      let emotion = "sad";
    }
    console.log("I am " + emotion);

    function MyComponent({}) {
      let [pressed, setPressed] = useState(false);
    }

    google.charts.load('current', {
      packages: ['corechart'],
    });

To fix this error, fix the name of the function or variable:

    console.write("Hello, world!");
    const one = Math.sin(Math.pi/2);

Alternatively, declare the function or variable:

    function encrypt(data) {
      return data ^ 0x13371337;
    }

Alternatively, move the declaration of the variable into an outer scope:

    let emotion;
    if (isHappy) {
      emotion = "happy";
    } else {
      emotion = "sad";
    }
    console.log("I am " + emotion);

Alternatively, import the function or variable:

    import { useState } from "react";
    function MyComponent({}) {
      let [pressed, setPressed] = useState(false);
    }

Alternatively, if the function or variable is global in your environment, write a quick-lint-js.config file.

Introduced in quick-lint-js version 0.2.0.

Documentation for other errors