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# `when()` The when method will execute the given callback when the first argument given to the method evaluates to true: ```js const collection = collect([1, 2, 3]); collection.when(true, items => items.push(4)); collection.all(); // [1, 2, 3, 4] ``` [View source on GitHub](https://github.com/ecrmnn/collect.js/blob/master/src/methods/when.js)