diff --git a/README.md b/README.md
index d6bec49..7bb89de 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ async function foo<T>(msg: T): void {
 }
 ```
 
-One more example that `/// to-promis-all` converts a sequence of `await` expressions to `await Promise.all()`:
+One more example that `/// to-promise-all` converts a sequence of `await` expressions to `await Promise.all()`:
 
 <!-- eslint-skip -->
 
diff --git a/src/commands/to-one-line.md b/src/commands/to-one-line.md
new file mode 100644
index 0000000..02ac078
--- /dev/null
+++ b/src/commands/to-one-line.md
@@ -0,0 +1,25 @@
+# `to-one-line`
+
+Convert multiple lines object to one line object.
+
+## Triggers
+
+- `/// to-one-line`
+- `/// tol`
+- `/// 21l`
+
+## Examples
+
+```js
+/// to-one-line
+const foo = {
+  bar: 1,
+  baz: 2
+}
+```
+
+Will be converted to:
+
+```js
+const foo = { bar: 1, baz: 2 }
+```
diff --git a/src/commands/to-one-line.test.ts b/src/commands/to-one-line.test.ts
new file mode 100644
index 0000000..59dd3b3
--- /dev/null
+++ b/src/commands/to-one-line.test.ts
@@ -0,0 +1,146 @@
+import { $, run } from './_test-utils'
+import { toOneLine as command } from './to-one-line'
+
+run(
+  command,
+  {
+    code: $`
+      /// to-one-line
+      const foo = {
+        bar: 1,
+        baz: 2,
+      }
+    `,
+    output: $`
+      const foo = { bar: 1, baz: 2 }
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      const arr = [
+        1,
+        2,
+        3,
+        4,
+      ]
+    `,
+    output: $`
+      const arr = [1, 2, 3, 4]
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// tol
+      obj = {
+        x: 100,
+        y: 200,
+      }
+    `,
+    output: $`
+      obj = { x: 100, y: 200 }
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      const data = {
+        user: {
+          name: 'Alice',
+          age: 30,
+        },
+        scores: [
+          10,
+          20,
+          30,
+        ],
+      }
+    `,
+    output: $`
+      const data = { user: { name: 'Alice', age: 30 }, scores: [10, 20, 30] }
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// 21l
+      const alreadyOneLine = { a: 1, b: 2 }
+    `,
+    output: $`
+      const alreadyOneLine = { a: 1, b: 2 }
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      const fruits = [
+        "apple",
+        "banana",
+        "cherry",
+      ]
+    `,
+    output: $`
+      const fruits = ["apple", "banana", "cherry"]
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      whichFruitIsTheBest([
+        "apple",
+        "banana",
+        "cherry",
+      ])
+    `,
+    output: $`
+      whichFruitIsTheBest(["apple", "banana", "cherry"])
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      function whichFruitIsTheBest({
+        apple,
+        banana,
+        cherry,
+      }) {}
+    `,
+    output: $`
+      function whichFruitIsTheBest({ apple, banana, cherry }) {}
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      function f([
+        a,
+        b,
+        c,
+      ]) {}
+    `,
+    output: $`
+      function f([a, b, c]) {}
+    `,
+    errors: ['command-fix'],
+  },
+  {
+    code: $`
+      /// to-one-line
+      return {
+        foo: 1,
+        bar: 2,
+      }
+    `,
+    output: $`
+      return { foo: 1, bar: 2 }
+    `,
+    errors: ['command-fix'],
+  },
+)
diff --git a/src/commands/to-one-line.ts b/src/commands/to-one-line.ts
new file mode 100644
index 0000000..5a1ce83
--- /dev/null
+++ b/src/commands/to-one-line.ts
@@ -0,0 +1,87 @@
+import type { Command, NodeType, Tree } from '../types'
+
+export const toOneLine: Command = {
+  name: 'to-one-line',
+  match: /^[/@:]\s*(?:to-one-line|21l|tol)$/,
+  action(ctx) {
+    const node = ctx.findNodeBelow(
+      'VariableDeclaration',
+      'AssignmentExpression',
+      'CallExpression',
+      'FunctionDeclaration',
+      'FunctionExpression',
+      'ReturnStatement',
+    )
+    if (!node)
+      return ctx.reportError('Unable to find node to convert')
+
+    let target: Tree.Node | null = null
+
+    // For a variable declaration we use the initializer.
+    if (node.type === 'VariableDeclaration') {
+      const decl = node.declarations[0]
+      if (decl && decl.init && isAllowedType(decl.init.type))
+        target = decl.init
+    }
+    // For an assignment we use the right side.
+    else if (node.type === 'AssignmentExpression') {
+      if (node.right && isAllowedType(node.right.type))
+        target = node.right
+    }
+    // In a call we search the arguments.
+    else if (node.type === 'CallExpression') {
+      target = node.arguments.find(arg => isAllowedType(arg.type)) || null
+    }
+    // In a function we search the parameters.
+    else if (
+      node.type === 'FunctionDeclaration'
+      || node.type === 'FunctionExpression'
+    ) {
+      target = node.params.find(param => isAllowedType(param.type)) || null
+    }
+    // For a return statement we use its argument.
+    else if (node.type === 'ReturnStatement') {
+      if (node.argument && isAllowedType(node.argument.type))
+        target = node.argument
+    }
+
+    if (!target)
+      return ctx.reportError('Unable to find object/array literal or pattern to convert')
+
+    // Get the text of the node to reformat it.
+    const original = ctx.getTextOf(target)
+    // Replace line breaks with spaces and remove extra spaces.
+    let oneLine = original.replace(/\n/g, ' ').replace(/\s{2,}/g, ' ').trim()
+    // Remove a comma that comes before a closing bracket or brace.
+    oneLine = oneLine.replace(/,\s*([}\]])/g, '$1')
+
+    if (target.type === 'ArrayExpression' || target.type === 'ArrayPattern') {
+      // For arrays, add a missing space before a closing bracket.
+      oneLine = oneLine.replace(/\[\s+/g, '[').replace(/\s+\]/g, ']')
+    }
+    else {
+      // For objects, add a missing space before a closing bracket or brace.
+      oneLine = oneLine.replace(/([^ \t])([}\]])/g, '$1 $2')
+      // Add a space between a ']' and a '}' if they touch.
+      oneLine = oneLine.replace(/\](\})/g, '] $1')
+    }
+
+    // Fix any nested array formatting.
+    oneLine = oneLine.replace(/\[\s+/g, '[').replace(/\s+\]/g, ']')
+
+    ctx.report({
+      node: target,
+      message: 'Convert object/array to one line',
+      fix: fixer => fixer.replaceTextRange(target.range, oneLine),
+    })
+
+    function isAllowedType(type: NodeType): boolean {
+      return (
+        type === 'ObjectExpression'
+        || type === 'ArrayExpression'
+        || type === 'ObjectPattern'
+        || type === 'ArrayPattern'
+      )
+    }
+  },
+}