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LESSON 122 OF 348 · WHILE LOOPS & FOR LOOPS

Looping in Reverse with a for Loop in TypeScript

Lesson Read the full explanation

A for loop can also count backwards, as long as we define the right conditions. For example:

for (let i = 7; i > 2; i--) {
	console.log(i);
}

The output of this code is:

7
6
5
4
3

Let's break down this code:

  • The initialization statement is let i = 7, so we start iterating from number 7.

  • We loop as long as i > 2, so we'll end at number 3. We could've written i >= 3 and it would still be correct.

  • The iteration statement is i--, so we decrease our number by 1 every time.

Assignment

Let's print all numbers from 5 through -5 in decreasing order.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 2
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    TYPESCRIPT · WHILE LOOPS & FOR LOOPS

    Looping In Reverse in TypeScript, explained

    Counting backwards

    A for loop does not have to count up. If you start at the high end, subtract in the update and test with a "greater than" comparison, the loop counts down. Reverse loops are useful whenever the order matters: a countdown, the last items first, or a walk from the end of an array to its start.

    The three changes

    Compared with a counting-up loop, three parts of the header change:

    for (let i = 7; i > 2; i--) {
    console.log(i);
}

    7
6
5
4
3
    • Initialization: let i = 7 starts at the largest value.
    • Condition: i > 2 keeps going while i is above 2, so the last value printed is 3. Writing i >= 3 gives the same loop.
    • Iteration: i-- subtracts 1 after each pass. It is the opposite of i++, and i -= 1 does the same.

    The comparison flips from < to > because the counter now approaches its limit from above. Keeping the old < would give a loop that never runs: 7 < 2 is false at the first check.

    Negative numbers

    Counting down through zero needs no special handling. Numbers below zero behave like any others, and the loop variable can be negative:

    for (let i = 2; i >= -2; i--) {
    console.log(i);
}

    This prints 2, 1, 0, -1 and -2. The same comparison works on both sides of zero, so there is no need to split the loop into a positive and a negative part.

    The exercise

    The task is to print every number from 5 down to -5, in decreasing order. That is 11 numbers: 5, 4, 3, 2, 1, 0, -1, -2, -3, -4, -5.

    1. Start at the top. Declare i with let and the type number, and start it at 5.
    2. Keep going while i >= -5. Using >= rather than > is what lets -5 itself be printed. The negative sign belongs to the number, so the comparison reads "at least -5".
    3. Decrease by 1 with i--.
    4. Print i inside the body with console.log.

    The solution in TypeScript

    for (let i: number = 5; i >= -5; i--) {
    console.log(i);
}

    After the pass that prints -5, the update makes i equal to -6, the condition -6 >= -5 is false, and the loop ends. The count of lines is first minus last, plus one: 5 - (-5) + 1 = 11.

    Looping over an array backwards

    The same pattern walks an array from its last element to its first. The last valid index is length - 1, and the first is 0:

    const letters: string[] = ["a", "b", "c"];
for (let i = letters.length - 1; i >= 0; i--) {
    console.log(letters[i]);
}

    This prints c, b and a. A start of letters.length would be one too high: the first pass would read an index past the end of the array, and TypeScript would give undefined without complaint.

    Common mistakes

    • Using i++ in a countdown. The counter then moves away from the limit and the condition stays true forever.
    • Keeping the old comparison. i <= -5 is false from the start for a start value of 5, so nothing is printed.
    • Stopping one step early. With i > -5 the output ends at -4. The task says "through -5", so -5 must be included.
    • Mixing up the order of the bounds. A reverse loop starts at the large value and ends at the small one. Reading the header aloud helps: "start at 5, keep going while at least -5, subtract 1 each time."

    Key takeaways

    • To count down, start high, use > or >= in the condition and subtract in the update.
    • The loop works the same way across zero and with negative numbers.
    • Use >= when the lowest value must be included.
    • For an array, a reverse loop starts at length - 1 and ends at index 0.
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