How to fix ScriptException: compile error in Elasticsearch

{
  "error": {
    "type": "script_exception",
    "reason": "compile error",
    "script_stack": [
      "... if (ctx.field == 'value') {",
      "                    ^---- HERE"
    ],
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "invalid sequence of tokens near ['value']."
    }
  }
}

// Wrong
def x = 1
def y = 2

// Correct
def x = 1;
def y = 2;

// Wrong
if (ctx.status == 'active')
  ctx.count++

// Correct
if (ctx.status == 'active') {
  ctx.count++;
}

// Wrong (single equals in condition)
if (ctx.field = 'value') { }

// Correct (double equals)
if (ctx.field == 'value') { }

POST /my-index/_explain/1
{
  "query": {
    "script_score": {
      "query": { "match_all": {} },
      "script": {
        "source": "Debug.explain(doc)"
      }
    }
  }
}

POST /_scripts/painless/_execute
{
  "script": {
    "source": "return 1 + 1;"
  }
}

POST /_scripts/painless/_execute
{
  "script": {
    "source": """
      def x = params.value;
      if (x > 10) {
        return x * 2;
      }
      return x;
    """,
    "params": {
      "value": 15
    }
  }
}

// Use doc['field'].value for doc values
doc['price'].value * 1.1

// Use params._source for source fields
params._source.nested.field

// Handle missing fields
doc.containsKey('field') ? doc['field'].value : 0

// Correct ternary syntax
def result = condition ? value1 : value2;

// Wrong - 'in' is a reserved keyword
def in = 5;

// Correct - use different name
def input = 5;