fix: bug when throttled function is sync

fix #11

src/index.js

@@ -32,7 +32,7 @@ class Delay {
 }
 
 function throttle<Args: Array<any>, Value>(
-  fn: (...args: Args) => Promise<Value>,
+  fn: (...args: Args) => Value | Promise<Value>,
   _wait: ?number,
   options: {
     getNextArgs?: (args0: Args, args1: Args) => Args,
@@ -52,10 +52,16 @@ function throttle<Args: Array<any>, Value>(
 
   function invoke(): Promise<Value> {
     const args = nextArgs
+    // istanbul ignore next
     if (!args) throw new Error('unexpected error: nextArgs is null')
     nextInvocation = null
     nextArgs = null
-    const result = fn(...args)
+    let result
+    try {
+      result = Promise.resolve(fn(...args))
+    } catch (error) {
+      result = Promise.reject(error)
+    }
     lastInvocationDone = result.catch(() => {})
     delay = new Delay(lastInvocationDone, wait)
     return result

test/index.js

@@ -72,7 +72,43 @@ describe('throttle', () => {
       getNextArgs: () => (null: any),
     })
     fn(1)
-    expect(() => fn(1)).to.throw()
+    expect(() => fn(1)).to.throw('unexpected error: nextArgs is null')
+  })
+  it('works with sync function', async function(): Promise<void> {
+    const foo = sinon.spy(
+      (a: number, wait?: number): number => {
+        if (a < 0) throw new Error()
+        return a * 2
+      }
+    )
+    const fn = throttle(foo, 100)
+    let promises = [fn(1), fn(2), fn(3)].map(wrapPromise)
+    for (let promise of promises) assert(promise.isPending())
+
+    await clock.tickAsync(1)
+    for (let promise of promises) expect(promise.value()).to.equal(6)
+    expect(foo.args).to.deep.equal([[3]])
+
+    promises = [fn(4), fn(-4), fn(5)].map(wrapPromise)
+    promises.forEach(p => p.catch(() => {}))
+    await clock.tickAsync(40)
+    for (let promise of promises) assert(promise.isPending())
+
+    await clock.tickAsync(60)
+    for (let promise of promises) expect(promise.value()).to.equal(10)
+
+    expect(foo.args).to.deep.equal([[3], [5]])
+
+    await clock.tickAsync(1000)
+    expect(foo.args).to.deep.equal([[3], [5]])
+
+    promises = [fn(1), fn(2), fn(3)].map(wrapPromise)
+    await clock.tickAsync(1)
+    for (let promise of promises) expect(promise.value()).to.equal(6)
+
+    promises = [fn(-10)].map(wrapPromise)
+    await clock.tickAsync(100)
+    assert(promises[0].isRejected())
   })
   it('works', async function(): Promise<void> {
     const foo = sinon.spy(