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A single-line comment in TypeScript is a lexical construct initiated by a double forward-slash (//) or triple forward-slash (///) that extends to the next line terminator (such as \n or \r) or the End of File (EOF). While standard single-line comments act as non-executable text, the TypeScript compiler explicitly parses specific single-line comments as pragmas or directives that directly alter type-checking evaluation, diagnostic reporting, and module resolution. Standard single-line comments are scanned and processed during the lexical analysis and parsing phases. During the emit phase, the compiler (tsc) merely determines whether to strip or preserve them in the generated JavaScript output based on the removeComments boolean flag in the tsconfig.json configuration. 
// Standard single-line comment preceding a statement
const isCompiled: boolean = true; 

const compilerTarget: string = "ES6"; // Inline single-line comment
Compiler Directives Unlike standard JavaScript, TypeScript evaluates specific single-line comments as compiler instructions rather than ignored text. These are categorized into type-checking directives and triple-slash directives. Type-Checking Directives Initiated by // followed by specific @ts- flags, these comments alter the compiler’s diagnostic behavior for the subsequent line or the entire file:
  • // @ts-expect-error: Suppresses the compiler error on the following line, but emits a new error if no type error actually exists.
  • // @ts-ignore: Unconditionally suppresses any compiler error on the following line.
  • // @ts-check: Enables standard type-checking for a JavaScript (.js) file (equivalent to enabling the checkJs compiler option). Whether the checking is strict depends on the strict family of compiler options in the configuration.
  • // @ts-nocheck: Disables type-checking for the entire file.
// @ts-expect-error
const count: number = "string"; 

// @ts-ignore
const data: boolean = 1;

// @ts-nocheck
// Disables checking for this entire file, preventing errors on the following line
const uncheckedVariable: number = "string";

// @ts-check
// Enables type-checking for this specific .js file
let checkedVariable = 10;
Triple-Slash Directives A specialized single-line comment initiated by /// containing a single XML tag. These must be placed at the top of the file (prior to any statements) and instruct the compiler on dependency resolution, type declarations, and compilation context. 
/// <reference path="internal-types.d.ts" />
/// <reference types="node" />
/// <reference lib="es2019.array" />
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