diff --git a/README.md b/README.md
index 5aa47a5..a96c84f 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # Diff-Match-Patch Unicode [![JSR](https://jsr.io/badges/@clearlylocal/diff-match-patch-unicode)](https://jsr.io/@clearlylocal/diff-match-patch-unicode)
 
-Modern JS/TS and Unicode-friendly version of [Neil Fraser](https://github.com/NeilFraser)’s [`diff-match-patch`](https://github.com/google/diff-match-patch).
+Modern JS/TS and Unicode-friendly version of [Neil Fraser](https://github.com/NeilFraser)’s [`diff-match-patch`](https://github.com/google/diff-match-patch). Currently only supports diffing; matching and patching may be added in the future, depending on need.
 
 ## Usage
 
diff --git a/src/Diff.test.ts b/src/Diff.test.ts
new file mode 100644
index 0000000..db556f6
--- /dev/null
+++ b/src/Diff.test.ts
@@ -0,0 +1,55 @@
+import { assert, assertEquals } from '@std/assert'
+import { Diff, DiffOperation } from './Diff.ts'
+
+Deno.test(Diff.name, async (t) => {
+	await t.step('TypeScript validates inputs', () => {
+		;(() => {
+			new Diff(0, 'a')
+			new Diff(1, 'a')
+			new Diff(-1, 'a')
+
+			// @ts-expect-error wrong order
+			new Diff('a', 1)
+			// @ts-expect-error 2 is not a valid op
+			new Diff(2, 'a')
+			// @ts-expect-error -2 is not a valid op
+			new Diff(-2, 'a')
+		})()
+	})
+
+	await t.step('diff.op', () => {
+		assertEquals(new Diff(DiffOperation.Delete, 'a').op, -1)
+		assertEquals(new Diff(DiffOperation.Equal, 'a').op, 0)
+		assertEquals(new Diff(DiffOperation.Insert, 'a').op, 1)
+	})
+
+	await t.step('other getters', () => {
+		const diff = new Diff(DiffOperation.Equal, 'a')
+		assertEquals(diff.text, 'a')
+		assertEquals(diff.length, 2)
+	})
+
+	await t.step('JSON.stringify', () => {
+		const diff = new Diff(DiffOperation.Equal, 'a')
+		assertEquals(JSON.stringify(diff), '[0,"a"]')
+	})
+
+	await t.step('iterate', () => {
+		const diff = new Diff(DiffOperation.Equal, 'a')
+		assertEquals([...diff], [0, 'a'])
+	})
+
+	await t.step('inspect', () => {
+		const diff = new Diff(DiffOperation.Equal, 'a')
+		assertEquals(Deno.inspect(diff, { colors: false }), 'Diff #[ 0, "a" ]')
+		assertEquals(Deno.inspect(diff, { colors: true }), 'Diff #[ \x1b[33m0\x1b[39m, \x1b[32m"a"\x1b[39m ]')
+	})
+
+	await t.step('clone', () => {
+		const diff = new Diff(DiffOperation.Equal, 'a')
+		// value-equal
+		assertEquals(diff, diff.clone())
+		// not reference-equal
+		assert(diff !== diff.clone())
+	})
+})
diff --git a/src/Differ.ts b/src/Differ.ts
index 58cc27d..be4b60a 100644
--- a/src/Differ.ts
+++ b/src/Differ.ts
@@ -137,14 +137,14 @@ export class Differ {
 	 * ```
 	 */
 	diff(before: string, after: string, options?: Partial<DiffOptions>): Diff[] {
-		if (before === after) {
-			// no need to go any further if both strings are the same
-			return before ? [new Diff(DiffOperation.Equal, before)] : []
-		}
-
 		const opts = { ...defaultDiffOptions, ...options, maxBefore: MAX_SEGMENTS_2_3, maxAfter: MAX_SEGMENTS }
 		const { join } = opts
 
+		if (before === after && join) {
+			// no need to go any further if both strings are the same (unless `join` is false)
+			return before ? [new Diff(DiffOperation.Equal, before)] : []
+		}
+
 		const { encodedDiffs, decode } = this.#diffInternal(before, after, opts)
 
 		if (!join) {
diff --git a/src/_DiffMatchPatch.ts b/src/_DiffMatchPatch.ts
index 92f1d5c..d074d2a 100644
--- a/src/_DiffMatchPatch.ts
+++ b/src/_DiffMatchPatch.ts
@@ -61,7 +61,7 @@ export class DiffMatchPatch {
 			if (this.Diff_Timeout <= 0) {
 				opt_deadline = Number.MAX_VALUE
 			} else {
-				opt_deadline = (new Date()).getTime() + this.Diff_Timeout * 1000
+				opt_deadline = Date.now() + this.Diff_Timeout * 1000
 			}
 		}
 		const deadline = opt_deadline
@@ -279,7 +279,7 @@ export class DiffMatchPatch {
 		let k2end = 0
 		for (let d = 0; d < max_d; d++) {
 			// Bail out if deadline is reached.
-			if ((new Date()).getTime() > deadline) {
+			if (Date.now() > deadline) {
 				break
 			}
 
diff --git a/src/utils.test.ts b/src/utils.test.ts
new file mode 100644
index 0000000..27e3b20
--- /dev/null
+++ b/src/utils.test.ts
@@ -0,0 +1,27 @@
+import { assert, assertInstanceOf, assertThrows } from '@std/assert'
+import { makeDiff } from './utils.ts'
+import { Diff } from './Diff.ts'
+
+Deno.test(makeDiff.name, async (t) => {
+	await t.step('instanceof', () => {
+		assertInstanceOf(makeDiff([0, 'a']), Diff)
+	})
+
+	await t.step('returns `Diff` input unchanged', () => {
+		const diff = makeDiff([0, 'a'])
+		// reference-equal
+		assert(diff === diff)
+	})
+
+	await t.step('validates inputs at runtime', () => {
+		makeDiff([0, 'a'])
+		makeDiff([1, 'a'])
+		makeDiff([-1, 'a'])
+
+		assertThrows(() => makeDiff([1, 1]), Error, 'Invalid text')
+		assertThrows(() => makeDiff(['a', '1']), Error, 'Invalid op')
+		assertThrows(() => makeDiff(['a', 1]), Error, 'Invalid text')
+		assertThrows(() => makeDiff([2, 'a']), Error, 'Invalid op')
+		assertThrows(() => makeDiff([-2, 'a']), Error, 'Invalid op')
+	})
+})
diff --git a/src/utils.ts b/src/utils.ts
index 15268a4..3c7820b 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,4 +1,4 @@
-import { Diff, DiffOperation } from './Diff.ts'
+import { Diff } from './Diff.ts'
 import { assert } from '@std/assert/assert'
 
 /**
@@ -19,7 +19,7 @@ export function makeDiffs(arr: readonly DiffLike[]): Diff[] {
 export function makeDiff(d: DiffLike): Diff {
 	if (d instanceof Diff) return d
 	const [op, text] = d
-	assert(typeof text === 'string')
-	assert(op === DiffOperation.Delete || op === DiffOperation.Insert || op === DiffOperation.Equal)
+	assert(typeof text === 'string', `Invalid text: type is ${typeof text}; expected string`)
+	assert(op === -1 || op === 0 || op === 1, `Invalid op: ${op}; expected -1, 0, or 1`)
 	return new Diff(op, text)
 }