feat(useTrunc): new function (#1838)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

packages/math/index.ts

@@ -14,3 +14,4 @@ export * from './useMin'
 export * from './useProjection'
 export * from './useRound'
 export * from './useSum'
+export * from './useTrunc'

packages/math/useTrunc/index.md

@@ -0,0 +1,18 @@
+---
+category: '@Math'
+---
+
+# useTrunc
+
+Reactive `Math.trunc`.
+
+## Usage
+
+```ts
+import { useTrunc } from '@vueuse/math'
+
+const value1 = ref(0.95)
+const value2 = ref(-2.34)
+const result1 = useTrunc(value1) // 0
+const result2 = useTrunc(value2) // -2
+```

packages/math/useTrunc/index.test.ts

@@ -0,0 +1,44 @@
+import { ref } from 'vue-demi'
+import { useTrunc } from '.'
+
+// Returns:
+//  0        ->  0
+// -0        -> -0
+//  0.2      ->  0
+// -0.2      -> -0
+//  0.7      ->  0
+// -0.7      -> -0
+//  Infinity ->  Infinity
+// -Infinity -> -Infinity
+//  NaN      ->  NaN
+//  null     ->  0
+
+describe('useTrunk', () => {
+  it('should be defined', () => {
+    expect(useTrunc).toBeDefined()
+  })
+  it('should work', () => {
+    const base = ref(1.95)
+    const result = useTrunc(base)
+    expect(result.value).toBe(1)
+    base.value = -7.004
+    expect(result.value).toBe(-7)
+
+    base.value = 0
+    expect(result.value).toBe(0)
+    base.value = -0
+    expect(result.value).toBe(-0)
+
+    base.value = 0.2
+    expect(result.value).toBe(0)
+    base.value = -0.2
+    expect(result.value).toBe(-0)
+
+    base.value = Infinity
+    expect(result.value).toBe(Infinity)
+    base.value = -Infinity
+    expect(result.value).toBe(-Infinity)
+    base.value = NaN
+    expect(result.value).toBe(NaN)
+  })
+})

packages/math/useTrunc/index.ts

@@ -0,0 +1,13 @@
+import type { ComputedRef } from 'vue-demi'
+import { computed } from 'vue-demi'
+import type { MaybeComputedRef } from '@vueuse/shared'
+import { resolveUnref } from '@vueuse/shared'
+
+/**
+ * Reactive `Math.trunc`.
+ *
+ * @see https://vueuse.org/useTrunc
+ */
+export function useTrunc(value: MaybeComputedRef<number>): ComputedRef<number> {
+  return computed<number>(() => Math.trunc(resolveUnref(value)))
+}