first

연산자(operator) 정의: first(predicate: function, select: function)

Emit the first value or first to pass provided expression.

The counterpart to first is last!

Examples

( example tests )

Example 1: First value from sequence

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const source = Rx.Observable.from([1,2,3,4,5]);
//no arguments, emit first value
const example = source.first();
//output: "First value: 1"
const subscribe = example.subscribe(val => console.log(`First value: ${val}`));

Example 2: First value to pass predicate

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const source = Rx.Observable.from([1,2,3,4,5]);
//emit first item to pass test
const example = source.first(num => num === 5);
//output: "First to pass test: 5"
const subscribe = example.subscribe(val => console.log(`First to pass test: ${val}`));

Example 3: Using optional projection function

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const source = Rx.Observable.from([1,2,3,4,5]);
//using optional projection function
const example = source.first(num => num % 2 === 0, 
                                    (result, index) => `First even: ${result} is at index: ${index}`);
//output: "First even: 2 at index: 1"
const subscribe = example.subscribe(val => console.log(val));

Example 4: Utilizing default value

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const source = Rx.Observable.from([1,2,3,4,5]);
//no value will pass, emit default
const example = source.first(val => val > 5, val => `Value: ${val}`, 'Nothing');
//output: 'Nothing'
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

• first - Official docs
• Filtering operator: take, first, skip - André Staltz

Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/operator/first.ts