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The while statement creates a control flow loop that executes a specified block of code repeatedly as long as a given test condition evaluates to true. Because the condition is evaluated before the execution of the loop body, it is classified as a pre-test loop. 
while (condition) {
  // statement(s)
}

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Execution Mechanics

  1. Condition Evaluation: The condition expression is evaluated before each iteration. JavaScript applies implicit boolean type coercion to the result.
  2. Truthy Path: If the condition evaluates to a truthy value, the statement block executes.
  3. Falsy Path: If the condition evaluates to a falsy value (e.g., false, 0, "", null, undefined, NaN), the loop terminates immediately. Control flow passes to the first statement following the while block.
  4. Iteration: After the statement block completes, control jumps back to Step 1.
Because it is a pre-test loop, if the condition evaluates to a falsy value on the very first check, the loop body will execute exactly zero times.

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Basic Implementation

let iteration = 0;

while (iteration < 3) {
  console.log(iteration);
  iteration++; 
}
// Output: 0, 1, 2
In this structure, the variable evaluated in the condition (iteration) must be mutated within the loop body. Failing to alter the state of the condition variable results in an infinite loop, blocking the main thread.

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Loop Control Statements

You can alter the standard execution flow of a while loop using specific control statements:
  • break: Immediately terminates the loop entirely, bypassing the condition check, and transfers control to the next statement outside the loop.
  • continue: Immediately halts the current iteration. Control jumps directly back to the condition evaluation to determine if the next iteration should begin.
let count = 0;

while (count < 5) {
  count++;
  
  if (count === 2) {
    continue; // Skips the rest of the block, jumps to condition
  }
  
  if (count === 4) {
    break; // Terminates the loop entirely
  }
  
  console.log(count);
}
// Output: 1, 3

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Type Coercion in Conditions

The while loop does not require a strict boolean primitive. It relies on JavaScript’s internal truthiness evaluation. 
let val = 3;

// The integer 3 is truthy. 
// The loop runs until val is decremented to 0 (which is falsy).
while (val) {
  val--;
}
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