Your first JavaScript program

The idea

A program is a sequence of instructions. Open the developer console in a browser, enter a statement, and press Enter to run it. console.log(...) prints a value. Text goes inside matching quotes; a number such as 2 is written without quotes. A semicolon can mark the end of a statement. The console may also display a return value such as undefined; that is separate from the line printed by console.log.

A variable gives a value a name. Write let completed = 2 to create a value you plan to change, then completed = completed + 1 to store its next value. The equals sign here means assignment: evaluate the right side, then save that result under the name on the left. Use const for a name you will not reassign. Identifier spelling and capitalization must match each time.

Read statements from top to bottom and track each name on paper. Numbers can be added; quoted text remains text. A boolean is either true or false and can describe a condition such as whether a target was reached. The next chapter develops comparisons and missing values. First predict the three output lines below, then run the program and compare.

const title = "Practice";
let completed = 2;
console.log(title);
completed = completed + 1;
console.log(completed);
console.log(completed >= 3);

Practice
3
true

The title is text. The assignment changes completed from two to three, and the comparison is true.