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How to test a value x against predicate function and returns fn(x) or x in JavaScript?
Testing a value x against a predicate function means checking if x meets a specific condition. If the condition is true, we apply a transformation function to x and return the result. Otherwise, we return the original value x unchanged.
Syntax
The basic syntax uses the ternary operator to test a value against a predicate function:
let result = predicate(x) ? operation(x) : x;
Where predicate(x) returns true/false, and operation(x) transforms the value when the predicate passes.
Basic Example
Here's a simple example that squares numbers greater than 5, otherwise returns the original value:
<html>
<body>
<h3>Testing a value x against predicate function</h3>
<div id="content"></div>
<script>
let content = document.getElementById("content");
function operation(x) {
return x * x;
}
function predicate(x) {
return x > 5;
}
let x = 10;
let result = predicate(x) ? operation(x) : x;
content.innerHTML = "Original value: " + x + ", Result: " + result + "<br>";
let x2 = 3;
let result2 = predicate(x2) ? operation(x2) : x2;
content.innerHTML += "Original value: " + x2 + ", Result: " + result2 + "<br>";
</script>
</body>
</html>
Advanced Example with Function Parameters
We can create a reusable test function that accepts predicate and operation functions as parameters:
<html>
<body>
<h3>Flexible predicate testing with function parameters</h3>
<div id="output"></div>
<script>
let output = document.getElementById("output");
// Operation functions
function halve(x) {
return x / 2;
}
function makePositive(x) {
return Math.abs(x);
}
// Predicate functions
function isEven(x) {
return x % 2 === 0;
}
function isNegative(x) {
return x < 0;
}
// Generic test function
function test(x, predicate, operation) {
output.innerHTML += "Testing value: " + x + "<br>";
let result = predicate(x) ? operation(x) : x;
output.innerHTML += "Result: " + result + "<br><br>";
return result;
}
// Test cases
test(6, isEven, halve); // 6 is even, so halve it ? 3
test(5, isEven, halve); // 5 is odd, return original ? 5
test(-8, isNegative, makePositive); // -8 is negative, make positive ? 8
test(10, isNegative, makePositive); // 10 is positive, return original ? 10
</script>
</body>
</html>
Common Use Cases
| Predicate | Operation | Use Case |
|---|---|---|
| x > threshold | clamp(x) | Limit maximum values |
| x < 0 | Math.abs(x) | Convert negative to positive |
| isString(x) | x.toUpperCase() | Transform strings only |
Conclusion
Using predicates with the ternary operator provides a clean way to conditionally transform values in JavaScript. This pattern is useful for validation, data transformation, and implementing conditional logic in a functional programming style.
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