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The error pyppeteer.errors.ElementHandleError: Evaluation failed: SyntaxError: Unexpected token return means the JavaScript sent for evaluation contains a top-level return. JavaScript permits return only inside a function body. In the reported requests-html example, wrap the code in an arrow function and pass that complete function to render():
script = """() => {
return Highcharts.charts[0].series[0].data.map(d => d.y);
}"""
chartdata = resp.html.render(script=script, reload=False)This changes the input from an invalid top-level statement into a function expression whose return value the browser can send back to Python.
Why “Unexpected token return” happens
Pyppeteer evaluates JavaScript in a browser page. The browser parser sees return before it has entered a function, so it reports Unexpected token return. The problem is the form of the JavaScript input, not the value returned by Highcharts or another page script.
A top-level expression such as Highcharts.charts[0].series[0].data.map(d => d.y) can be valid JavaScript. A top-level return Highcharts... is not valid JavaScript because there is no enclosing function to return from.
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Invalid top-level form
return Highcharts.charts[0].series[0].data.map(d => d.y);Valid function form
() => {
return Highcharts.charts[0].series[0].data.map(d => d.y);
}The distinction matters because wrappers do not necessarily parse a string in the same way. The accepted answer for the reported case demonstrates the arrow-function form for requests-html; it does not prove that every wrapper internally wraps strings identically.
Fix the reported requests-html call
-
Build a complete function expression, including
() => {and the closing brace. -
Keep the
returninside that function body. -
Pass the function string to
resp.html.render(script=..., reload=False).
from requests_html import HTMLSession
session = HTMLSession()
resp = session.get("https://example.com/chart")
script = """() => {
return Highcharts.charts[0].series[0].data.map(d => d.y);
}"""
chartdata = resp.html.render(script=script, reload=False)
print(chartdata)Replace the URL with your page. The page must have loaded Highcharts and populated Highcharts.charts[0]; otherwise the syntax error will be gone but a later runtime error may occur because the object does not exist.
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reload=False tells requests-html not to reload the page while rendering. It does not change JavaScript grammar and cannot make a top-level return legal. Keep it when you already have the desired page state; remove or change it only when your page requires a fresh navigation before the script runs.
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Expression versus function input
Evaluation APIs commonly support one or both of these shapes:
| Input shape | Example | When to use it |
|---|---|---|
| Expression | 1 + 2 |
Use when the API explicitly evaluates a JavaScript expression and you do not need a return statement. |
| Function | () => { return 1 + 2; } |
Use when the API expects a callable function or when you need statements, local variables, conditionals, or an explicit return. |
Pyppeteer 0.0.25 documents Page.evaluate as executing a JavaScript function or expression and returning the result. Its force_expr option, which defaults to False, controls expression treatment: force_expr=True forces expression mode. See the Pyppeteer 0.0.25 reference for the exact method signature and behavior.
If you are calling direct Pyppeteer rather than requests-html, check that method’s accepted input before changing the script. A string that works through one wrapper may be parsed differently by another.
Direct Pyppeteer examples
Evaluate a function
import asyncio
from pyppeteer import launch
async def main():
browser = await launch()
page = await browser.newPage()
await page.goto("https://example.com/chart", {"waitUntil": "networkidle2"})
values = await page.evaluate("""() => {
return Highcharts.charts[0].series[0].data.map(d => d.y);
}""")
print(values)
await browser.close()
asyncio.get_event_loop().run_until_complete(main())Passing the function form makes the return context explicit and is generally easier to debug. Current Puppeteer documentation likewise describes Page.evaluate as accepting a function or string and recommends a function for easier debugging; its page identifies Puppeteer 25.12.0. That current JavaScript documentation is a comparison, not evidence that an older Python wrapper has identical internals. See Puppeteer Page.evaluate documentation.
Evaluate an expression instead
values = await page.evaluate(
"Highcharts.charts[0].series[0].data.map(d => d.y)",
force_expr=True,
)Do not combine expression mode with a bare return. If you force expression mode, send the expression itself.
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A reliable debugging sequence
- Identify the actual caller. Determine whether the failing line is
resp.html.render(script=...),page.evaluate(...), or another abstraction. The accepted fix is specifically demonstrated for therequests-htmlcall. - Print the exact JavaScript string. Hidden concatenation, indentation, or an accidentally truncated closing brace can change what reaches Chromium.
- Choose one input shape. Use
() => { ... }with an internalreturn, or use a return-free expression. Do not paste a function body beginning withreturnwhere a complete function is required. - Reduce the page expression. First evaluate
typeof Highcharts, thenHighcharts.charts.length, then the data mapping. This separates syntax problems from page-state problems. - Capture the complete traceback. The first syntax correction may reveal a different error, such as an undefined object, a missing chart, or a navigation failure.
checks = await page.evaluate("""() => ({
highchartsType: typeof Highcharts,
chartCount: typeof Highcharts === 'undefined' ? 0 : Highcharts.charts.length
})""")
print(checks)Common follow-up failures after the syntax fix
Highcharts is not defined
The chart library has not loaded in the page context, or the page uses a different charting library. Wait for the relevant script or inspect the page’s actual global names. This is a runtime/page-state issue, not the original parser error.
Cannot read properties of undefined
Highcharts.charts[0] may not exist yet. A navigation can finish before an application renders its chart. Wait for a chart-specific selector or poll for the object before evaluating the mapping.
The result is null or an empty array
The function executed, but the chart has no matching points at that moment. Confirm that the series is populated and that the page did not replace the chart after your evaluation.
The browser fails before evaluation
Record the Python package version, Pyppeteer version, Chromium revision, operating system, navigation URL, and full traceback. Pyppeteer 0.0.25 says it works best with its bundled Chromium and does not guarantee compatibility with other Chromium versions. A browser-version mismatch can produce failures unrelated to JavaScript syntax.
Version and wrapper checks
Use the environment that actually runs the failing code when diagnosing compatibility:
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python -m pip show pyppeteer requests-html
python --versionAlso log the Chromium executable or revision selected by your installation. Do not assume that direct Pyppeteer, requests-html, and modern Puppeteer share the same argument parsing or browser support. The documented case establishes a working wrapper-specific function form, while the Pyppeteer reference documents the direct API’s function/expression options.
What not to change
- Do not remove
returnfrom a multi-statement function merely to silence the parser; you may then lose the value you need. - Do not add
force_expr=Trueto a wrapper that does not expose that option. - Do not diagnose a missing chart, blocked navigation, or incompatible Chromium as an “unexpected token” problem after the JavaScript parses successfully.
- Do not assume a string accepted by one evaluation layer is passed unchanged to another layer.
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Does adding parentheses around return fix the error?
No. Parentheses alone do not create a function. Pass a complete function such as () => { return value; }, or pass a return-free expression when the API supports expression input.
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Can I use an async arrow function?
Yes, when the evaluation API supports promises: use async () => { return await ...; }. Confirm that your specific wrapper awaits the returned promise.
Is this error caused by Highcharts?
Not in the reported case. The parser rejects the top-level return before Highcharts data is read. Highcharts loading or chart timing can cause separate runtime errors after the syntax is corrected.
Should I upgrade Pyppeteer immediately?
Not solely for this message. First correct the JavaScript input shape and identify your caller. Upgrade or change Chromium only after recording versions and investigating a separate compatibility or browser-launch failure.
The Bottom Line
Put return inside a function passed to the evaluation API. For the reported requests-html call, the working shape is () => { return ...; }; if the error persists, inspect the exact caller, JavaScript string, page state, and Chromium versions.
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