Input and output

Programs display results and sometimes receive user input. To preserve the output–formatting–input–practice progression used by the C, C++, and Java courses, this lesson uses the browser console's console.log() and the browser-provided prompt() function.

Output with console.log

console.log() writes one or more values to the developer console. Put literal strings in quotes; do not quote a variable name when you want its value. In a string, \n is a newline, \t is a tab, \" is a double quote, and \\ is a backslash.

const course = "JavaScript";
console.log("Hello World!");
console.log("Course:", course);
console.log("\"Hello\nWorld\"");

Joining and formatting values

A template literal uses backticks and inserts a value with ${expression}. Call toFixed() when a number needs a fixed count of decimal places; it returns a formatted string.

const length = 10;
const width = 5;
const area = length * width;
console.log(`Area: ${length} × ${width} = ${area.toFixed(2)}`);

Input with prompt

In a browser, prompt() opens an input dialog and returns a string; Cancel returns null. Typing 5 still produces "5". Use Number() before arithmetic and Number.isNaN() to detect a failed conversion.

const rawRadius = prompt("Radius:");
const radius = Number(rawRadius);

if (Number.isNaN(radius)) {
  console.log("Enter a valid number");
} else {
  const area = Math.PI * radius * radius;
  console.log(`Area: ${area.toFixed(2)}`);
}

prompt is a browser Web API; Node.js uses different APIs for terminal input. The Lab uses a repeatable simulated interface rather than opening real dialogs.