I have ./src/app.ts with code

function one() {
    let result;
    function two() {
        console.log(result);
        console.log("one");
    }
}

package.json

{
  "name": "hello_world",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "nodemon ./src/app.ts",
    "build": "tsc"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "nodemon": "^3.1.7",
    "typescript": "^5.7.2"
  }}

in tsconfig.json

"noEmitOnError": true,

when i run npm run build

i see ./public/app.js

"use strict";
function one() {
    let result;
    function two() {
        console.log(result);
        console.log("one");
    }
}

why code compiled even though result is undefined.

but in my vscode red sviggly line for result variable

This new behavior is actually the 1st item of the 5.7 release announcement.

Checks for Never-Initialized Variables

For a long time, TypeScript has been able to catch issues when a variable has not yet been initialized in all prior branches.

let result: number
if (someCondition()) {
    result = doSomeWork();
}
else {
    let temporaryWork = doSomeWork();
    temporaryWork *= 2;
    // forgot to assign to 'result'
}

console.log(result); // error: Variable 'result' is used before being assigned.

Unfortunately, there are some places where this analysis doesn’t work. For example, if the variable is accessed in a separate function, the type system doesn’t know when the function will be called, and instead takes an "optimistic" view that the variable will be initialized.

function foo() {
    let result: number
    if (someCondition()) {
        result = doSomeWork();
    }
    else {
        let temporaryWork = doSomeWork();
        temporaryWork *= 2;
        // forgot to assign to 'result'
    }

    printResult();

    function printResult() {
        console.log(result); // no error here.
    }
}

While TypeScript 5.7 is still lenient on variables that have possibly been initialized, the type system is able to report errors when variables have never been initialized at all.

function foo() {
    let result: number
    
    // do work, but forget to assign to 'result'

    function printResult() {
        console.log(result); // error: Variable 'result' is used before being assigned.
    }
}