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Implementing loops in LESS

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This article looks at how to implement for loops in LESS, which is harder than it would seem unfortunately.

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The LESS pre-processor doesn’t actually have a for loop, or even a while loop. Instead it has the ability to simulate them with a feature called guard expressions, which is a mixin that will only execute if a given condition passes.

.optional-style(@switch) when (@switch) {
  color: black;
}

button {
  /* Add optional styles */
  @switch: true;
  .optional-style(@switch);
}

li {
  /* Don't add optional styles */
  @switch: false;
  .optional-style(@switch);
}

A for loop can be simulated by using a guard expression that calls itself with the loop index decremented.

.my-loop(@i) when (@i > 0) {
  .my-loop(@i - 1);
  li:nth-child(@{i}) {
    color:red;
  }
}

/* Run the loop */
.my-loop(5);

/* Produces li:nth-child(1) ... li:nth-child(5) */

To reverse the order in which the loop is evaluated, place the recursive call at the other end of the guard expression.

.my-loop(@i) when (@i > 0) {
  li:nth-child(@{i}) {
    color:red;
  }
  .my-loop(@i - 1);
}

/* Run the loop */
.my-loop(5);

/* Produces li:nth-child(5) ... li:nth-child(1) */

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