Polymorphic this types
A polymorphic this type represents a type that is the subtype of the containing class or interface. This is called F-bounded polymorphism. This makes hierarchical fluent interfaces much easier to express, for example. Take a simple calculator that returns this after each operation:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | class BasicCalculator { public constructor(protected value: number = 0) { } public currentValue(): number { return this.value; } public add(operand: number): this { this.value += operand; return this; } public multiply(operand: number): this { this.value *= operand; return this; } // ... other operations go here ...}let v = new BasicCalculator(2) .multiply(5) .add(1) .currentValue(); |
Since the class uses this types, you can extend it and the new class can use the old methods with no changes.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | class ScientificCalculator extends BasicCalculator { public constructor(value = 0) { super(value); } public sin() { this.value = Math.sin(this.value); return this; } // ... other operations go here ...}let v = new ScientificCalculator(2) .multiply(5) .sin() .add(1) .currentValue(); |
Without this types, ScientificCalculator would not have been able to extend BasicCalculator and keep the fluent interface. multiply would have returned BasicCalculator, which doesn’t have the sin method. However, with this types, multiply returns this, which is ScientificCalculator here.
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