✨ Add length function

common/length.ts

@@ -0,0 +1,20 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+/**
+ * Returns length property.
+ *
+ * @param val - Value with length property
+ * @returns The result of `val.length`
+ *
+ * @example
+ * ```ts
+ * length('hello') // 5
+ * length(['hello', 'world', 1]) // 3
+ * length({length: 5, text: 'hello'}) // 5
+ * ```
+ *
+ * @beta
+ */
+const length = <T extends { length: number }>(val: T): T["length"] =>
+  val.length;
+
+export { length };

common/length_test.ts

@@ -0,0 +1,28 @@
+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+import { assertEquals, assertEqualsType } from "../dev_deps.ts";
+import { length } from "./length.ts";
+
+Deno.test("length", () => {
+  const table: [{ length: number } | string, number][] = [
+    ["", 0],
+    [" ", 1],
+    ["  ", 2],
+    ["a", 1],
+    ["ab", 2],
+    ["hello", 5],
+    [[], 0],
+    [[""], 1],
+    [["", 0], 2],
+    [["", 0, "1"], 3],
+    [{ length: 3 }, 3],
+  ];
+
+  table.forEach(([val, expected]) => {
+    assertEquals(length(val), expected, `length(${val}) -> ${expected}`);
+  });
+
+  assertEqualsType<number>(length(""));
+  assertEqualsType<number>(length([]));
+  assertEqualsType<number>(length({ length: 1 }));
+  assertEqualsType<1>(length({ length: 1 } as const));
+});

common/mod.ts

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+// Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
+export { length } from "./length.ts";

mod.ts

@@ -1,2 +1,3 @@
 // Copyright 2021-present the Core-fn authors. All rights reserved. MIT license.
 export * from "./string/mod.ts";
+export * from "./common/mod.ts";