diff --git a/packages/backend/src/misc/acct.ts b/packages/backend/src/misc/acct.ts
index 5b7767a106..cb6808b4b4 100644
--- a/packages/backend/src/misc/acct.ts
+++ b/packages/backend/src/misc/acct.ts
@@ -4,7 +4,7 @@ export type Acct = {
 };
 
 export function parse(acct: string): Acct {
-	if (acct.startsWith("@")) acct = acct.substr(1);
+	if (acct.startsWith("@")) acct = acct.slice(1);
 	const split = acct.split("@", 2);
 	return { username: split[0], host: split[1] || null };
 }
diff --git a/packages/backend/src/server/api/service/discord.ts b/packages/backend/src/server/api/service/discord.ts
index 9906d2f7ca..296e9cb481 100644
--- a/packages/backend/src/server/api/service/discord.ts
+++ b/packages/backend/src/server/api/service/discord.ts
@@ -18,7 +18,7 @@ function getUserToken(ctx: Koa.BaseContext): string | null {
 
 function compareOrigin(ctx: Koa.BaseContext): boolean {
 	function normalizeUrl(url?: string): string {
-		return url ? (url.endsWith("/") ? url.substr(0, url.length - 1) : url) : "";
+		return slice( url.length - 1) : url) : "";
 	}
 
 	const referer = ctx.headers["referer"];
diff --git a/packages/backend/src/server/api/service/github.ts b/packages/backend/src/server/api/service/github.ts
index f77c5f795d..da9d0931a0 100644
--- a/packages/backend/src/server/api/service/github.ts
+++ b/packages/backend/src/server/api/service/github.ts
@@ -18,7 +18,7 @@ function getUserToken(ctx: Koa.BaseContext): string | null {
 
 function compareOrigin(ctx: Koa.BaseContext): boolean {
 	function normalizeUrl(url?: string): string {
-		return url ? (url.endsWith("/") ? url.substr(0, url.length - 1) : url) : "";
+		return slice( url.length - 1) : url) : "";
 	}
 
 	const referer = ctx.headers["referer"];
diff --git a/packages/calckey-js/src/acct.ts b/packages/calckey-js/src/acct.ts
index 5b7767a106..cb6808b4b4 100644
--- a/packages/calckey-js/src/acct.ts
+++ b/packages/calckey-js/src/acct.ts
@@ -4,7 +4,7 @@ export type Acct = {
 };
 
 export function parse(acct: string): Acct {
-	if (acct.startsWith("@")) acct = acct.substr(1);
+	if (acct.startsWith("@")) acct = acct.slice(1);
 	const split = acct.split("@", 2);
 	return { username: split[0], host: split[1] || null };
 }
diff --git a/packages/client/src/components/MkUserPreview.vue b/packages/client/src/components/MkUserPreview.vue
index 6c3838fd2e..7aec661368 100644
--- a/packages/client/src/components/MkUserPreview.vue
+++ b/packages/client/src/components/MkUserPreview.vue
@@ -155,7 +155,7 @@ onMounted(() => {
 			user.description.length > 400;
 	} else {
 		const query = props.q.startsWith("@")
-			? Acct.parse(props.q.substr(1))
+			? Acct.parse(props.q.slice(1))
 			: { userId: props.q };
 
 		os.api("users/show", query).then((res) => {
diff --git a/packages/client/src/scripts/gen-search-query.ts b/packages/client/src/scripts/gen-search-query.ts
index 822572d3e3..fcf1f727ea 100644
--- a/packages/client/src/scripts/gen-search-query.ts
+++ b/packages/client/src/scripts/gen-search-query.ts
@@ -8,7 +8,7 @@ export async function genSearchQuery(v: any, q: string) {
 		for (const at of q
 			.split(" ")
 			.filter((x) => x.startsWith("@"))
-			.map((x) => x.substr(1))) {
+			.map((x) => x.slice(1))) {
 			if (at.includes(".")) {
 				if (at === localHost || at === ".") {
 					host = null;
diff --git a/packages/client/src/scripts/search.ts b/packages/client/src/scripts/search.ts
index 34318ab0e9..ecea1ff2d4 100644
--- a/packages/client/src/scripts/search.ts
+++ b/packages/client/src/scripts/search.ts
@@ -19,7 +19,7 @@ export async function search() {
 	}
 
 	if (q.startsWith("#")) {
-		mainRouter.push(`/tags/${encodeURIComponent(q.substr(1))}`);
+		mainRouter.push(`/tags/${encodeURIComponent(q.slice(1))}`);
 		return;
 	}
 
diff --git a/packages/client/src/scripts/theme-editor.ts b/packages/client/src/scripts/theme-editor.ts
index 4828410177..fc015b6ac2 100644
--- a/packages/client/src/scripts/theme-editor.ts
+++ b/packages/client/src/scripts/theme-editor.ts
@@ -35,7 +35,7 @@ export const fromThemeString = (str?: string): ThemeValue => {
 	} else if (str.startsWith('"')) {
 		return {
 			type: "css",
-			value: str.substr(1).trim(),
+			value: str.slice(1).trim(),
 		};
 	} else {
 		return str;
diff --git a/packages/client/src/scripts/theme.ts b/packages/client/src/scripts/theme.ts
index 383deb34ce..c52c2c217c 100644
--- a/packages/client/src/scripts/theme.ts
+++ b/packages/client/src/scripts/theme.ts
@@ -112,7 +112,7 @@ function compile(theme: Theme): Record<string, string> {
 	function getColor(val: string): tinycolor.Instance {
 		// ref (prop)
 		if (val[0] === "@") {
-			return getColor(theme.props[val.substr(1)]);
+			return getColor(theme.props[val.slice(1)]);
 		}
 
 		// ref (const)
@@ -123,7 +123,7 @@ function compile(theme: Theme): Record<string, string> {
 		// func
 		else if (val[0] === ":") {
 			const parts = val.split("<");
-			const func = parts.shift().substr(1);
+			const func = parts.shift().slice(1);
 			const arg = parseFloat(parts.shift());
 			const color = getColor(parts.join("<"));