Allow disabling leading check (#14)

Author: Richienb

index.d.ts

@@ -13,6 +13,15 @@ declare namespace pWaitFor {
 		@default Infinity
 		*/
 		readonly timeout?: number;
+
+		/**
+		Whether to run the check immediately rather than starting by waiting `interval` milliseconds.
+
+		Useful for when the check, if run immediately, would likely return `false`. In this scenario, set `leadingCheck` to `false`.
+
+		@default true
+		*/
+		readonly leadingCheck?: boolean;
 	}
 }
 

index.js

@@ -5,6 +5,7 @@ const pWaitFor = async (condition, options) => {
 	options = {
 		interval: 20,
 		timeout: Infinity,
+		leadingCheck: true,
 		...options
 	};
 
@@ -29,7 +30,11 @@ const pWaitFor = async (condition, options) => {
 			}
 		};
 
-		check();
+		if (options.leadingCheck) {
+			check();
+		} else {
+			retryTimeout = setTimeout(check, options.interval);
+		}
 	});
 
 	if (options.timeout !== Infinity) {

index.test-d.ts

@@ -5,3 +5,4 @@ expectType<Promise<void>>(pWaitFor(() => false));
 expectType<Promise<void>>(pWaitFor(() => Promise.resolve(false)));
 expectType<Promise<void>>(pWaitFor(() => true, {interval: 1}));
 expectType<Promise<void>>(pWaitFor(() => true, {timeout: 1}));
+expectType<Promise<void>>(pWaitFor(() => true, {leadingCheck: false}));

readme.md

@@ -55,6 +55,14 @@ Default: `Infinity`
 
 Number of milliseconds to wait before automatically rejecting.
 
+##### leadingCheck
+
+Type: `boolean`\
+Default: `true`
+
+Whether to run the check immediately rather than starting by waiting `interval` milliseconds.
+
+Useful for when the check, if run immediately, would likely return `false`. In this scenario, set `leadingCheck` to `false`.
 
 ## Related
 

test.js

@@ -58,3 +58,15 @@ test('stops performing checks if a timeout occurs', async t => {
 			t.is(checksPerformed, checksAtTimeout);
 		});
 });
+
+test('does not perform a leading check', async t => {
+	const ms = 200;
+	const end = timeSpan();
+
+	await pWaitFor(async () => true, {
+		interval: ms,
+		leadingCheck: false
+	});
+
+	t.true(end() > (ms - 20));
+})